diff --git a/download.js b/download.js
index 37110d01..83f28e23 100644
--- a/download.js
+++ b/download.js
@@ -1,5 +1,6 @@
 var downloadLogger, jqueryUis,
 	_ = require( "underscore" ),
+	Builder = require( "./lib/builder" ),
 	fs = require( "fs" ),
 	Handlebars = require( "handlebars" ),
 	JqueryUi = require( "./lib/jquery-ui" ),
@@ -83,7 +84,7 @@ Frontend.prototype = {
 
 	create: function( fields, response, callback ) {
 		try {
-			var build, components, packer, start, theme,
+			var builder, components, jqueryUi, packer, start, theme,
 				themeVars = null;
 			if ( fields.theme !== "none" ) {
 				themeVars = querystring.parse( fields.theme );
@@ -99,10 +100,11 @@ Frontend.prototype = {
 			});
 			components = Object.keys( _.omit( fields, "scope", "theme", "theme-folder-name", "version" ) );
 			start = new Date();
-			build = JqueryUi.find( fields.version ).build( components, {
+			jqueryUi = JqueryUi.find( fields.version );
+			builder = new Builder( jqueryUi, components, {
 				scope: fields.scope
 			});
-			packer = new Packer( build, theme, {
+			packer = new Packer( builder, theme, {
 				scope: fields.scope
 			});
 			response.setHeader( "Content-Type", "application/zip" );
@@ -116,9 +118,9 @@ Frontend.prototype = {
 					JSON.stringify({
 						build_size: written,
 						build_time: new Date() - start,
-						components: build.components,
+						components: builder.components,
 						theme_name: theme.name,
-						version: build.pkg.version
+						version: jqueryUi.pkg.version
 					})
 				);
 				return callback();
diff --git a/lib/builder.1.10.0.js b/lib/builder.1.10.0.js
index fb38796c..e9de886f 100644
--- a/lib/builder.1.10.0.js
+++ b/lib/builder.1.10.0.js
@@ -15,7 +15,7 @@ stripBanner = util.stripBanner;
 /**
  * Builder 1.10
  */
-function Builder_1_10_0( build, jqueryUi, components, options ) {
+function Builder_1_10_0( build, jqueryUi, components, options, callback ) {
 	var _bundleCss, baseCss, baseCssMin, cssComponentFileNames, existingCss, jsComponentFileNames, selectedDemoRe, selectedRe,
 		files = jqueryUi.files(),
 		min = function( file ) {
@@ -200,7 +200,7 @@ function Builder_1_10_0( build, jqueryUi, components, options ) {
 	// Ad hoc
 	build.jqueryCore = files.jqueryCore;
 
-	return build;
+	callback( null, build );
 }
 
 module.exports = Builder_1_10_0;
diff --git a/lib/builder.1.11.0.js b/lib/builder.1.11.0.js
index 0ef0f1fd..6b155208 100644
--- a/lib/builder.1.11.0.js
+++ b/lib/builder.1.11.0.js
@@ -1,9 +1,11 @@
 var _basename, demoIndexTemplate, docsTemplate, flatten, stripBanner,
+	async = require( "async" ),
 	banner = require( "./banner" ),
 	Files = require( "./files" ),
 	fs = require( "fs" ),
 	handlebars = require( "handlebars" ),
 	path = require( "path" ),
+	rjs = require( "./rjs" ),
 	sqwish = require( "sqwish" ),
 	ThemeRoller = require( "./themeroller" ),
 	util = require( "./util" );
@@ -23,8 +25,8 @@ path.basename = function() {
 /**
  * Builder 1.11
  */
-function Builder_1_11_0( build, jqueryUi, components, options ) {
-	var _bundleCss, baseCss, baseCssMin, cssComponentFileNames, docsCategories, existingCss, jsComponentFileNames, selectedDemoRe, selectedRe,
+function Builder_1_11_0( build, jqueryUi, components, options, callback ) {
+	var _bundleCss, baseCss, baseCssMin, cssComponentFileNames, docsCategories, existingCss, selectedDemoRe, selectedRe,
 		files = jqueryUi.files(),
 		min = function( file ) {
 			return files.min( file );
@@ -69,41 +71,6 @@ function Builder_1_11_0( build, jqueryUi, components, options ) {
 		return (/images/).test( file.path );
 	});
 
-	// I18n files
-	if ( components.indexOf( "datepicker" ) >= 0 ) {
-		build.i18nFiles = files.i18nFiles;
-		build.i18nMinFiles = files.i18nFiles.map( min );
-		build.bundleI18n = Files({
-			path: "jquery-ui-i18n.js",
-			data: files.i18nFiles.reduce(function( sum, file ) {
-				return sum + stripBanner( file );
-			}, banner( jqueryUi.pkg, files.i18nFiles.paths().map( path.basename ) ) )
-		});
-		build.bundleI18nMin = Files({
-			path: "jquery-ui-i18n.min.js",
-			data: banner( jqueryUi.pkg, files.i18nFiles.paths().map( path.basename ), { minify: true }) + stripBanner( files.min( build.bundleI18n[ 0 ] ) )
-		});
-	} else {
-		build.i18nFiles = build.i18nMinFiles = build.bundleI18n = build.bundleI18nMin = Files();
-	}
-
-	// Bundle JS (and minified)
-	jsComponentFileNames = components.map(function( component ) {
-		return component + ".js";
-	});
-	build.bundleJs = Files({
-		path: "jquery-ui.js",
-		data: build.components.reduce(function( sum, component ) {
-			return sum + stripBanner( files.get( "ui/" + component + ".js" ) );
-		}, banner( jqueryUi.pkg, jsComponentFileNames ) )
-	});
-	build.bundleJsMin = Files({
-		path: "jquery-ui.min.js",
-		data: build.components.reduce(function( sum, component ) {
-			return sum + stripBanner( files.min( files.get( "ui/" + component + ".js" ) ) );
-		}, banner( jqueryUi.pkg, jsComponentFileNames, { minify: true } ) )
-	});
-
 	// Bundle CSS (and minified)
 	existingCss = function( component ) {
 		return files.get( "themes/base/" + component + ".css" ) !== undefined;
@@ -208,7 +175,84 @@ function Builder_1_11_0( build, jqueryUi, components, options ) {
 	// Ad hoc
 	build.jqueryCore = files.jqueryCore;
 
-	return build;
+	// I18n files
+	function i18nFiles( callback ) {
+		if ( components.indexOf( "datepicker" ) >= 0 ) {
+			build.i18nFiles = files.i18nFiles;
+			build.i18nMinFiles = files.i18nFiles.map( min );
+			async.series([
+				function( callback ) {
+					rjs({
+						files: files,
+						include: files.i18nFiles.rename( /ui\//, "" ).map(function( file ) {
+							return file.path;
+						}),
+						exclude: [ "jquery", "jqueryui/core", "jqueryui/datepicker" ],
+						jquery: files.jqueryCore[ 0 ].path
+					}, function( error, data ) {
+						if ( error ) {
+							return callback( error );
+						}
+						build.bundleI18n = Files({
+							path: "jquery-ui-i18n.js",
+							data: banner( jqueryUi.pkg, files.i18nFiles.paths().map( path.basename ) ) + data
+						});
+						callback();
+					});
+				},
+				function( callback ) {
+					build.bundleI18nMin = Files({
+						path: "jquery-ui-i18n.min.js",
+						data: banner( jqueryUi.pkg, files.i18nFiles.paths().map( path.basename ), { minify: true }) + stripBanner( files.min( build.bundleI18n[ 0 ] ) )
+					});
+					callback();
+				}
+			], callback );
+		} else {
+			build.i18nFiles = build.i18nMinFiles = build.bundleI18n = build.bundleI18nMin = Files();
+			callback();
+		}
+	}
+
+	// Bundle JS (and minified)
+	function bundleJs( callback ) {
+		var jsComponentFileNames = components.map(function( component ) {
+			return component + ".js";
+		});
+		async.series([
+			function( callback ) {
+				rjs({
+					files: files,
+					include: jsComponentFileNames,
+					exclude: [ "jquery" ],
+					jquery: files.jqueryCore[ 0 ].path
+				}, function( error, data ) {
+					if ( error ) {
+						return callback( error );
+					}
+					build.bundleJs = Files({
+						path: "jquery-ui.js",
+						data: banner( jqueryUi.pkg, jsComponentFileNames ) + data
+					});
+					callback();
+				});
+			},
+			function( callback ) {
+				build.bundleJsMin = Files({
+					path: "jquery-ui.min.js",
+					data: banner( jqueryUi.pkg, jsComponentFileNames, { minify: true } ) + stripBanner( files.min( build.bundleJs[ 0 ] ) )
+				});
+				callback();
+			}
+		], callback );
+	}
+
+	async.series([
+		i18nFiles,
+		bundleJs
+	], function( error ) {
+		callback( error, build );
+	});
 }
 
 module.exports = Builder_1_11_0;
diff --git a/lib/builder.js b/lib/builder.js
index 1dd726c1..3fcec9a6 100644
--- a/lib/builder.js
+++ b/lib/builder.js
@@ -1,6 +1,4 @@
 var cache,
-	Builder_1_10_0 = require( "./builder.1.10.0.js" ),
-	Builder_1_11_0 = require( "./builder.1.11.0.js" ),
 	Cache = require( "./cache" ),
 	semver = require( "semver" );
 
@@ -22,23 +20,47 @@ function Builder( jqueryUi, components, options ) {
 }
 
 Builder.prototype = {
-	build: function() {
+	build: function( callback ) {
 		var cacheKey = this.jqueryUi.pkg.version + JSON.stringify( this.expandComponents( this.components ) ),
 			cached = cache.get( cacheKey );
 
 		if ( cached ) {
-			return cached;
+
+			// if we have data, call the callback, otherwise push ours
+			if ( cached.data ) {
+				callback( null, cached.data );
+			} else {
+				cached.callbacks.push( callback );
+			}
+			return true;
+		}
+
+		cached = {
+			callbacks: [ callback ]
+		};
+		cache.set( cacheKey, cached );
+
+		function done( err, data ) {
+			var callbacks = cached.callbacks;
+			if ( !err ) {
+				cached.data = data;
+				delete cached.callbacks;
+			}
+			callbacks.forEach(function( callback ) {
+				callback( err, data );
+			});
+			delete cached.callbacks;
+			if ( err ) {
+				cache.destroy( cacheKey );
+			}
 		}
 
 		// FIXME s/1.11.0pre/1.11.0
 		if ( semver.gte( this.jqueryUi.pkg.version, "1.11.0pre" ) ) {
-			cached = Builder_1_11_0( this, this.jqueryUi, this.components, this.options );
+			require( "./builder.1.11.0" )( this, this.jqueryUi, this.components, this.options, done );
 		} else {
-			cached = Builder_1_10_0( this, this.jqueryUi, this.components, this.options );
+			require( "./builder.1.10.0" )( this, this.jqueryUi, this.components, this.options, done );
 		}
-
-		cache.set( cacheKey, cached );
-		return cached;
 	},
 
 	expandComponents: function( components ) {
diff --git a/lib/jquery-ui.js b/lib/jquery-ui.js
index 6065768f..b2faec5b 100644
--- a/lib/jquery-ui.js
+++ b/lib/jquery-ui.js
@@ -87,15 +87,6 @@ JqueryUi.find = function( version ) {
 };
 
 JqueryUi.prototype = {
-	build: function( components, options ) {
-		var builder;
-		if ( !Builder ) {
-			Builder = require( "./builder" );
-		}
-		builder = new Builder( this, components, options );
-		return builder.build();
-	},
-
 	categories: function() {
 		if ( !this._categories ) {
 			var map = {};
diff --git a/lib/packer.js b/lib/packer.js
index 08859013..5a669045 100644
--- a/lib/packer.js
+++ b/lib/packer.js
@@ -23,8 +23,8 @@ function stripThemeImport( src ) {
 
 
 /**
- * Packer( build, theme, options )
- * - build [ instanceof Builder ]: jQuery UI build object.
+ * Packer( builder, theme, options )
+ * - builder [ instanceof Builder ]: jQuery UI builder object.
  * - theme [ instanceof ThemeRoller ]: theme object.
  * - options: details below.
  *
@@ -35,14 +35,14 @@ function stripThemeImport( src ) {
  * - skipDocs [ Boolean ]: Skip doc files on package. Default is false.
  * - skipTheme [ Boolean ]: Skip theme files on package. Default is false.
  */
-function Packer( build, theme, options ) {
+function Packer( builder, theme, options ) {
 	this.options = options = options || {};
 	if ( typeof options.bundleSuffix === "undefined" ) {
 		options.bundleSuffix = ".custom";
 	}
 
-	this.basedir = "jquery-ui-" + build.pkg.version + options.bundleSuffix;
-	this._build = build;
+	this.basedir = "jquery-ui-" + builder.jqueryUi.pkg.version + options.bundleSuffix;
+	this.builder = builder;
 	this.theme = theme;
 
 	if ( options.skipTheme ) {
@@ -74,7 +74,8 @@ Packer.prototype = {
 	 * }
 	 */
 	pack: function( callback ) {
-		var add = function( file ) {
+		var build,
+			add = function( file ) {
 				if ( arguments.length === 2 ) {
 					file = {
 						path: arguments[ 0 ],
@@ -87,162 +88,181 @@ Packer.prototype = {
 				});
 			},
 			basedir = this.basedir,
-			build = this._build,
+			builder = this.builder,
 			options = this.options,
 			output = [],
 			theme = this.theme;
 
-		// Common and component files (includes baseThemeFiles)
-		build.commonFiles.into( "development-bundle/" ).forEach( add );
-		build.componentFiles.into( "development-bundle/" ).forEach( add );
+		function _build( callback ) {
+			builder.build(function( error, _build ) {
+				if ( error ) {
+					return callback( error );
+				}
+				build = _build;
+				callback();
+			});
+		}
 
-		build.componentMinFiles
-			.rename( /^ui/, "ui/minified" )
-			.into( "development-bundle/" ).forEach( add );
+		function pack( callback ) {
+			// Common and component files (includes baseThemeFiles)
+			build.commonFiles.into( "development-bundle/" ).forEach( add );
+			build.componentFiles.into( "development-bundle/" ).forEach( add );
 
-		// Base theme (baseThemeFiles have been included by componentFiles)
-		build.baseThemeMinFiles
-			.rename( /^themes\/base/, "themes/base/minified" )
-			.into( "development-bundle/" ).forEach( add );
+			build.componentMinFiles
+				.rename( /^ui/, "ui/minified" )
+				.into( "development-bundle/" ).forEach( add );
 
-		build.baseThemeImages
-			.rename( /^themes\/base\/images/, "themes/base/minified/images" )
-			.into( "development-bundle/" ).forEach( add );
+			// Base theme (baseThemeFiles have been included by componentFiles)
+			build.baseThemeMinFiles
+				.rename( /^themes\/base/, "themes/base/minified" )
+				.into( "development-bundle/" ).forEach( add );
 
-		// I18n
-		build.i18nFiles.into( "development-bundle/" ).forEach( add );
-		build.i18nMinFiles
-			.rename( /^ui\/i18n/, "ui/minified/i18n" )
-			.into( "development-bundle/" ).forEach( add );
-		build.bundleI18n.into( "development-bundle/ui/i18n/" ).forEach( add );
-		build.bundleI18nMin.into( "development-bundle/ui/minified/i18n/" ).forEach( add );
+			build.baseThemeImages
+				.rename( /^themes\/base\/images/, "themes/base/minified/images" )
+				.into( "development-bundle/" ).forEach( add );
 
-		// Bundle JS (and minified)
-		build.bundleJs
-			.rename( /^jquery-ui/, "jquery-ui" + options.bundleSuffix )
-			.into( "development-bundle/ui/" ).forEach( add );
+			// I18n
+			build.i18nFiles.into( "development-bundle/" ).forEach( add );
+			build.i18nMinFiles
+				.rename( /^ui\/i18n/, "ui/minified/i18n" )
+				.into( "development-bundle/" ).forEach( add );
+			build.bundleI18n.into( "development-bundle/ui/i18n/" ).forEach( add );
+			build.bundleI18nMin.into( "development-bundle/ui/minified/i18n/" ).forEach( add );
 
-		build.bundleJs
-			.rename( /^jquery-ui/, "jquery-ui-" + build.pkg.version + options.bundleSuffix )
-			.into( "js/" ).forEach( add );
+			// Bundle JS (and minified)
+			build.bundleJs
+				.rename( /^jquery-ui/, "jquery-ui" + options.bundleSuffix )
+				.into( "development-bundle/ui/" ).forEach( add );
 
-		build.bundleJsMin
-			.rename( /^jquery-ui/, "jquery-ui" + options.bundleSuffix )
-			.into( "development-bundle/ui/minified/" ).forEach( add );
+			build.bundleJs
+				.rename( /^jquery-ui/, "jquery-ui-" + build.pkg.version + options.bundleSuffix )
+				.into( "js/" ).forEach( add );
 
-		build.bundleJsMin
-			.rename( /^jquery-ui/, "jquery-ui-" + build.pkg.version + options.bundleSuffix )
-			.into( "js/" ).forEach( add );
+			build.bundleJsMin
+				.rename( /^jquery-ui/, "jquery-ui" + options.bundleSuffix )
+				.into( "development-bundle/ui/minified/" ).forEach( add );
 
-		// Bundle CSS (and minified)
-		build.bundleCss( build.baseTheme ).into( "development-bundle/themes/base/" ).forEach( add );
-		build.bundleCssMin( build.baseThemeMin ).into( "development-bundle/themes/base/minified/" ).forEach( add );
-		if ( !this.options.skipTheme ) {
-			build.bundleCss( theme )
-				.tee(function( clone ) {
-					clone.into( "development-bundle/themes/" + theme.folderName() + "/" ).forEach( add );
-				})
+			build.bundleJsMin
 				.rename( /^jquery-ui/, "jquery-ui-" + build.pkg.version + options.bundleSuffix )
-				.into( "css/" + theme.folderName() + "/" ).forEach( add );
+				.into( "js/" ).forEach( add );
 
-			build.bundleCssMin( theme )
-				.tee(function( clone ) {
-					clone.into( "development-bundle/themes/" + theme.folderName() + "/minified/" ).forEach( add );
-				})
-				.rename( /^jquery-ui/, "jquery-ui-" + build.pkg.version + options.bundleSuffix )
-				.into( "css/" + theme.folderName() + "/" ).forEach( add );
-		}
+			// Bundle CSS (and minified)
+			build.bundleCss( build.baseTheme ).into( "development-bundle/themes/base/" ).forEach( add );
+			build.bundleCssMin( build.baseThemeMin ).into( "development-bundle/themes/base/minified/" ).forEach( add );
+			if ( !options.skipTheme ) {
+				build.bundleCss( theme )
+					.tee(function( clone ) {
+						clone.into( "development-bundle/themes/" + theme.folderName() + "/" ).forEach( add );
+					})
+					.rename( /^jquery-ui/, "jquery-ui-" + build.pkg.version + options.bundleSuffix )
+					.into( "css/" + theme.folderName() + "/" ).forEach( add );
 
-		// Demo files
-		build.demoFiles.into( "development-bundle/" ).forEach( add );
+				build.bundleCssMin( theme )
+					.tee(function( clone ) {
+						clone.into( "development-bundle/themes/" + theme.folderName() + "/minified/" ).forEach( add );
+					})
+					.rename( /^jquery-ui/, "jquery-ui-" + build.pkg.version + options.bundleSuffix )
+					.into( "css/" + theme.folderName() + "/" ).forEach( add );
+			}
 
-		// Doc files
-		if ( !options.skipDocs ) {
-			build.docFiles.into( "development-bundle/" ).forEach( add );
-		}
+			// Demo files
+			build.demoFiles.into( "development-bundle/" ).forEach( add );
 
-		// Test files
-		if ( options.addTests ) {
-			build.testFiles.into( "development-bundle/" ).forEach( add );
-		}
+			// Doc files
+			if ( !options.skipDocs ) {
+				build.docFiles.into( "development-bundle/" ).forEach( add );
+			}
+
+			// Test files
+			if ( options.addTests ) {
+				build.testFiles.into( "development-bundle/" ).forEach( add );
+			}
 
-		// Ad hoc
-		build.jqueryCore.into( "js/" ).forEach( add );
-		add( "index.html", indexTemplate({
-			bundleCss: "jquery-ui-" + build.pkg.version + options.bundleSuffix + ".css",
-			bundleJs: "jquery-ui-" + build.pkg.version + options.bundleSuffix + ".js",
-			jqueryCore: build.jqueryCore[ 0 ].path,
-			ui: build.components.reduce(function( sum, component ) {
-				sum[ component ] = true;
-				return sum;
-			}, {}),
-			themeFolderName: theme.folderName(),
-			version: build.pkg.version
-		}));
+			// Ad hoc
+			build.jqueryCore.into( "js/" ).forEach( add );
+			add( "index.html", indexTemplate({
+				bundleCss: "jquery-ui-" + build.pkg.version + options.bundleSuffix + ".css",
+				bundleJs: "jquery-ui-" + build.pkg.version + options.bundleSuffix + ".js",
+				jqueryCore: build.jqueryCore[ 0 ].path,
+				ui: build.components.reduce(function( sum, component ) {
+					sum[ component ] = true;
+					return sum;
+				}, {}),
+				themeFolderName: theme.folderName(),
+				version: build.pkg.version
+			}));
 
-		// Custom theme files
-		if ( !this.options.skipTheme ) {
-			build.baseThemeExceptThemeOrImages.forEach(function( file ) {
-				if ( theme.isNull && (/all.css/).test( file.path ) ) {
-					add( "development-bundle/themes/" + theme.folderName() + "/" + path.basename( file.path ), stripThemeImport( file.data ) );
+			// Custom theme files
+			if ( !options.skipTheme ) {
+				build.baseThemeExceptThemeOrImages.forEach(function( file ) {
+					if ( theme.isNull && (/all.css/).test( file.path ) ) {
+						add( "development-bundle/themes/" + theme.folderName() + "/" + path.basename( file.path ), stripThemeImport( file.data ) );
+					} else {
+						add( file.path.replace( "themes/base", "development-bundle/themes/" + theme.folderName() ), options.scope ? util.scope( file.data.toString( "utf8" ), options.scope ) : file.data );
+					}
+				});
+				// minified custom theme
+				build.baseThemeMinFiles.filter(function( file ) {
+					return !(/theme\W/).test( file.path );
+				}).map(function( file ) {
+					return options.scope ? { path: file.path, data: util.scope( file.data, options.scope ) } : file;
+				}).rename( /^themes\/base/, "themes/" + theme.folderName() + "/minified" ).into( "development-bundle/" ).forEach( add );
+			}
+			if ( !theme.isNull ) {
+				// FIXME s/1.11.0pre/1.11.0
+				if ( semver.gte( build.pkg.version, "1.11.0pre" ) ) {
+					add( "development-bundle/themes/" + theme.folderName() + "/theme.css", theme.css() );
+					add( "development-bundle/themes/" + theme.folderName() + "/minified/theme.min.css", banner( build.pkg, null, { minify: true } ) + sqwish.minify( util.stripBanner({ data: theme.css() }) ) );
 				} else {
-					add( file.path.replace( "themes/base", "development-bundle/themes/" + theme.folderName() ), options.scope ? util.scope( file.data.toString( "utf8" ), options.scope ) : file.data );
+					add( "development-bundle/themes/" + theme.folderName() + "/jquery.ui.theme.css", theme.css() );
+					add( "development-bundle/themes/" + theme.folderName() + "/minified/jquery.ui.theme.min.css", banner( build.pkg, null, { minify: true } ) + sqwish.minify( util.stripBanner({ data: theme.css() }) ) );
 				}
-			});
-			// minified custom theme
-			build.baseThemeMinFiles.filter(function( file ) {
-				return !(/theme\W/).test( file.path );
-			}).map(function( file ) {
-				return options.scope ? { path: file.path, data: util.scope( file.data, options.scope ) } : file;
-			}).rename( /^themes\/base/, "themes/" + theme.folderName() + "/minified" ).into( "development-bundle/" ).forEach( add );
-		}
-		if ( !theme.isNull ) {
-			// FIXME s/1.11.0pre/1.11.0
-			if ( semver.gte( build.pkg.version, "1.11.0pre" ) ) {
-				add( "development-bundle/themes/" + theme.folderName() + "/theme.css", theme.css() );
-				add( "development-bundle/themes/" + theme.folderName() + "/minified/theme.min.css", banner( build.pkg, null, { minify: true } ) + sqwish.minify( util.stripBanner({ data: theme.css() }) ) );
-			} else {
-				add( "development-bundle/themes/" + theme.folderName() + "/jquery.ui.theme.css", theme.css() );
-				add( "development-bundle/themes/" + theme.folderName() + "/minified/jquery.ui.theme.min.css", banner( build.pkg, null, { minify: true } ) + sqwish.minify( util.stripBanner({ data: theme.css() }) ) );
 			}
-		}
 
-		// Custom theme image files
-		if ( theme.isNull ) {
-			callback( null, output );
-		}
-		else {
-			async.series([
-				function( callback ) {
-					var themeImages = Files();
-					if ( semver.gte( build.pkg.version, "1.10.0" ) ) {
-						themeImages.push({
-							path: "animated-overlay.gif",
-							data: build.get( "themes/base/images/animated-overlay.gif" ).data
-						});
+			// Custom theme image files
+			if ( theme.isNull ) {
+				callback( null, output );
+			}
+			else {
+				async.series([
+					function( callback ) {
+						var themeImages = Files();
+						if ( semver.gte( build.pkg.version, "1.10.0" ) ) {
+							themeImages.push({
+								path: "animated-overlay.gif",
+								data: build.get( "themes/base/images/animated-overlay.gif" ).data
+							});
+						}
+						if ( themeImagesCache[ theme.name ] ) {
+							// Cached
+							callback( null, themeImages.concat( themeImagesCache[ theme.name ] ) );
+						} else {
+							// Not cached, fetch them
+							theme.generateImages(function( err, imageFiles ) {
+								if ( err ) {
+									callback( err, null );
+									return;
+								}
+								callback( null, themeImages.concat( imageFiles ) );
+							});
+						}
 					}
-					if ( themeImagesCache[ theme.name ] ) {
-						// Cached
-						callback( null, themeImages.concat( themeImagesCache[ theme.name ] ) );
-					} else {
-						// Not cached, fetch them
-						theme.generateImages(function( err, imageFiles ) {
-							if ( err ) {
-								callback( err, null );
-								return;
-							}
-							callback( null, themeImages.concat( imageFiles ) );
-						});
+				], function( err, themeImages ) {
+					if ( err ) {
+						callback( err, null );
 					}
-				}
-			], function( err, themeImages ) {
-				if ( err ) {
-					callback( err, null );
-				}
-				themeImages[ 0 ].into( [ "css/" + theme.folderName() + "/images/", "development-bundle/themes/" + theme.folderName() + "/images/", "development-bundle/themes/" + theme.folderName() + "/minified/images/" ] ).forEach( add );
-				callback( null, output );
-			});
+					themeImages[ 0 ].into( [ "css/" + theme.folderName() + "/images/", "development-bundle/themes/" + theme.folderName() + "/images/", "development-bundle/themes/" + theme.folderName() + "/minified/images/" ] ).forEach( add );
+					callback( null, output );
+				});
+			}
 		}
+
+		async.series([
+			_build,
+			pack
+		], function( error ) {
+			callback( error, output );
+		});
 	},
 
 	filename: function() {
diff --git a/lib/rjs.js b/lib/rjs.js
new file mode 100644
index 00000000..581e0729
--- /dev/null
+++ b/lib/rjs.js
@@ -0,0 +1,221 @@
+var fileApi, files, mutex, prim, rjs,
+	_ = require( "underscore" ),
+	fs = require( "fs" ),
+	logger = require( "simple-log" ).init( "download.jqueryui.com" ),
+	path = require( "path" ),
+	requirejs = require( "requirejs" );
+
+fileApi = {
+	backSlashRegExp: /\\/g,
+	exclusionRegExp: /^\./,
+
+	absPath: function(fileName) {
+		// path.charAt( 0 ) must be / or requirejs' nameToUrl will be calculated wrong.
+		return "/";
+	},
+
+	copyDir: function( srcDir, destDir, regExpFilter ) {
+		var destPaths;
+		srcDir = path.normalize( srcDir );
+		destDir = path.normalize( destDir );
+		destPaths = fileApi.getFilteredFileList( srcDir, regExpFilter ).map(function( src ) {
+			var dest = src.replace( srcDir, destDir );
+			fileApi.copyFile( src, dest );
+			return dest;
+		});
+		return destPaths.length ? destPaths : null;
+	},
+
+	copyFile: function( src, dest ) {
+		// Ignore root slash
+		src = src.substr( 1 );
+		dest = dest.substr( 1 );
+
+		files[ dest ] = {
+			path: dest,
+			data: files[ src ].data
+		};
+		return true;
+	},
+
+	deleteFile: function( path ) {
+		// Ignore root slash
+		path = path.substr( 1 );
+		delete files[ path ];
+	},
+
+	exists: function( path ) {
+		// Ignore root slash
+		path = path.substr( 1 );
+		return path in files;
+	},
+
+	getFilteredFileList: function( startDir, regExpFilters, makeUnixPaths ) {
+		var regExp, regExpInclude, regExpExclude;
+
+		regExpInclude = regExpFilters.include || regExpFilters;
+		regExpExclude = regExpFilters.exclude || null;
+
+		if ( regExpExclude ) {
+			throw new Error( "exclude filter not supported" );
+		}
+
+		regExp = new RegExp( path.join( startDir, ".*" ) + ( regExpInclude ).toString().replace( /^\//, "" ).replace( /\/$/, "" ) );
+
+		return Object.keys( files ).filter(function( path ) {
+			return regExp.test( "/" + path );
+		}).map(function( path ) {
+			return "/" + path;
+		});
+	},
+
+	normalize: function( fileName ) {
+		return path.normalize( fileName );
+	},
+
+	readFile: function( path ) {
+		// Ignore root slash
+		path = path.substr( 1 );
+
+		try {
+			return files[ path ].data.toString( "utf8" );
+		} catch ( err ) {
+			err.message = "File not found: " + path + "\n" + err.message;
+			throw err;
+		}
+	},
+
+	readFileAsync: function( path ) {
+		var deferred = prim();
+		try {
+			deferred.resolve( fileApi.readFile( path ) );
+		} catch ( error ) {
+			deferred.reject( error );
+		}
+		return deferred.promise;
+	},
+
+	renameFile: function( from, to ) {
+		from = path.normalize( from );
+		to = path.normalize( to );
+
+		fileApi.copyFile( from, to );
+
+		// Ignore root slash
+		from = from.substr( 1 );
+
+		delete files[ from ];
+		return true;
+	},
+
+	saveFile: function( _path, data ) {
+		_path = path.normalize( _path );
+
+		// Ignore root slash
+		_path = _path.substr( 1 );
+
+		files[ _path ] = {
+			path: _path,
+			data: data
+		};
+	},
+
+	saveUtf8File: function( fileName, fileContents ) {
+		fileApi.saveFile( fileName, fileContents );
+	}
+};
+
+rjs = function( attributes, callback ) {
+	var localCallback;
+
+	if ( mutex ) {
+		throw new Error( "Concurrent calls not supported" );
+	}
+	mutex = true;
+	localCallback = function( error, results ) {
+		mutex = false;
+		callback( error, results );
+	};
+
+	attributes = attributes || {};
+	if ( !( "files" in attributes ) ) {
+		throw new Error( "missing attributes.files" );
+	}
+	if ( !( "include" in attributes ) ) {
+		throw new Error( "missing attributes.include" );
+	}
+	if ( !( "jquery" in attributes ) ) {
+		throw new Error( "missing attributes.jquery" );
+	}
+	if ( !attributes.include.length ) {
+		return localCallback( null, "" );
+	}
+	attributes.exclude = attributes.exclude || [];
+
+	files = _.clone( attributes.files.cache );
+
+	fileApi.saveFile( "/dist/tmp/main.js", "require([\n\t\"jqueryui/" + attributes.include.map(function( filename ) {
+		return filename.replace( /\.js$/, "" );
+	}).join( "\",\n\t\"jqueryui/" ) + "\"\n]);" );
+
+	requirejs.define( "node/file", [ "prim" ], function( _prim ) {
+		prim = _prim;
+		return fileApi;
+	});
+
+	requirejs.define( "node/print", [], function() {
+		return function print( msg ) {
+			logger.log( msg );
+			if ( msg.substring( 0, 5 ) === "Error" ) {
+				throw new Error( msg );
+			}
+		};
+	});
+
+	requirejs.optimize({
+		dir: "dist/build",
+		appDir: "ui",
+		baseUrl: ".",
+		optimize: "none",
+		optimizeCss: "none",
+		paths: {
+			jquery: "../" + attributes.jquery.replace( /\.js$/, "" ),
+			jqueryui: ".",
+			tmp: "../dist/tmp"
+		},
+		modules: [{
+			name: "../output",
+			include: [ "tmp/main" ],
+			exclude: attributes.exclude,
+			create: true
+		}],
+		wrap: {
+			start: "(function( $ ) {",
+			end: "})( jQuery );"
+		},
+		logLevel: 2,
+		onBuildWrite: function( id, path, contents ) {
+			if ( (/define\([\s\S]*?factory/).test( contents ) ) {
+				// Remove UMD wrapper
+				contents = contents.replace( /\(function\( factory[\s\S]*?\(function\( \$ \) \{/, "" );
+				contents = contents.replace( /\}\)\);\s*?$/, "" );
+			}
+			else if ( (/^require\(\[/).test( contents ) ) {
+				// Replace require with comment `//` instead of null string, because of the mysterious semicolon
+				contents = contents.replace( /^require[\s\S]*?\]\);$/, "// mysterious semicolon: " );
+			}
+			return contents;
+		}
+	}, function() {
+
+		// Remove `define("main" ...)` and `define("jquery-ui" ...)`
+		var contents = fileApi.readFile( "/dist/output.js" ).replace( /define\("(tmp\/main|\.\.\/output)", function\(\)\{\}\);/g, "" );
+
+		// Remove the mysterious semicolon `;` character left from require([...]);
+		contents = contents.replace( /\/\/ mysterious semicolon.*/g, "" );
+
+		localCallback( null, contents );
+	}, localCallback );
+};
+
+module.exports = rjs;
diff --git a/package.json b/package.json
index f2a148dc..613c7e98 100644
--- a/package.json
+++ b/package.json
@@ -17,6 +17,7 @@
     "handlebars": "1.0.12",
     "lzma": "1.2.1",
     "optimist": "0.3.4",
+    "requirejs": "2.1.8",
     "rimraf": "2.0.2",
     "semver": "1.1.0",
     "simple-log": "1.1.0",
diff --git a/test/packer.js b/test/packer.js
index 406ad010..81441a9f 100644
--- a/test/packer.js
+++ b/test/packer.js
@@ -18,7 +18,7 @@ function filePresent( files, filepath ) {
 
 function pack( jqueryUi, components, theme, callback ) {
 	var builder = new Builder( jqueryUi, components ),
-		packer = new Packer( builder.build(), theme );
+		packer = new Packer( builder, theme );
 	packer.pack(function( err, files ) {
 		if ( err ) {
 			callback( err, null );
@@ -561,7 +561,7 @@ var tests = {
 			scope = "#wrapper";
 		test.expect( filesToCheck.length );
 		builder = new Builder( this.jqueryUi, components, { scope: scope } );
-		packer = new Packer( builder.build(), this.theme, { scope: scope } );
+		packer = new Packer( builder, this.theme, { scope: scope } );
 		packer.pack(function( err, files ) {
 			if ( err ) {
 				test.ok( false, err.message );
diff --git a/test/packer.legacy.1.10.js b/test/packer.legacy.1.10.js
index 0e7e2f63..a4afd2e0 100644
--- a/test/packer.legacy.1.10.js
+++ b/test/packer.legacy.1.10.js
@@ -18,7 +18,7 @@ function filePresent( files, filepath ) {
 
 function pack( jqueryUi, components, theme, callback ) {
 	var builder = new Builder( jqueryUi, components ),
-		packer = new Packer( builder.build(), theme );
+		packer = new Packer( builder, theme );
 	packer.pack(function( err, files ) {
 		if ( err ) {
 			callback( err, null );
@@ -536,7 +536,7 @@ var tests = {
 			scope = "#wrapper";
 		test.expect( filesToCheck.length );
 		builder = new Builder( this.jqueryUi, components, { scope: scope } );
-		packer = new Packer( builder.build(), this.theme, { scope: scope } );
+		packer = new Packer( builder, this.theme, { scope: scope } );
 		packer.pack(function( err, files ) {
 			if ( err ) {
 				test.ok( false, err.message );
diff --git a/test/packer.legacy.1.9.js b/test/packer.legacy.1.9.js
index c4217231..cbf2f349 100644
--- a/test/packer.legacy.1.9.js
+++ b/test/packer.legacy.1.9.js
@@ -18,7 +18,7 @@ function filePresent( files, filepath ) {
 
 function pack( jqueryUi, components, theme, callback ) {
 	var builder = new Builder( jqueryUi, components ),
-		packer = new Packer( builder.build(), theme );
+		packer = new Packer( builder, theme );
 	packer.pack(function( err, files ) {
 		if ( err ) {
 			callback( err, null );