Use position.source for mapping lines

Author: andreypopp

lib/compiler.js

@@ -30,7 +30,7 @@ Compiler.prototype.emit = function(str, pos, startOnly) {
         line: this.position.line,
         column: Math.max(this.position.column - 1, 0)
       },
-      source: 'source.css',
+      source: pos.source || 'source.css',
       original: {
         line: pos.start.line,
         column: pos.start.column - 1