Fix/restore working source map generation in 0.7.3

Author: dleffler

src/SourceMap/SourceMapGenerator.php

@@ -174,9 +174,17 @@ public function generateJson()
         $root = $this->options['sourceRoot'];
 
         if ($root) {
+//            $sourceMap['sourceRoot'] = $root;
+            // A list of symbol names used by the 'mappings' entry.
             $sourceMap['names'] = array();
         }
 
+        // A list of original sources used by the 'mappings' entry.
+        $sourceMap['sources'] = array();
+        foreach($this->sources as $source_uri => $source_filename) {
+            $sourceMap['sources'][] = $this->normalizeFilename($source_filename);
+        }
+
         // A string with the encoded mapping data.
         $sourceMap['mappings'] = $mappings;