feat(Grunt): add source maps to all min files

Generate source map files when build step is ran and adds source map
headers to all min files.

Source map url must be appended to the min file otherwise the line
offsets will be off.

Inspired by Ryan Seddon (PR #2858)

Closes #1714

lib/grunt/utils.js

@@ -112,27 +112,44 @@ module.exports = {
   },
 
 
-  singleStrict: function(src, insert, newline){
-    var useStrict = newline ? "$1\n'use strict';" : "$1'use strict';";
+  singleStrict: function(src, insert){
     return src
       .replace(/\s*("|')use strict("|');\s*/g, insert) // remove all file-specific strict mode flags
-      .replace(/(\(function\([^)]*\)\s*\{)/, useStrict); // add single strict mode flag
+      .replace(/(\(function\([^)]*\)\s*\{)/, "$1'use strict';"); // add single strict mode flag
+  },
+
+
+  sourceMap: function(mapFile, fileContents) {
+    // use the following once Chrome beta or stable supports the //# pragma
+    // var sourceMapLine = '//# sourceMappingURL=' + mapFile + '\n';
+    var sourceMapLine = '/*\n//@ sourceMappingURL=' + mapFile + '\n*/\n';
+    return fileContents + sourceMapLine;
   },
 
 
   min: function(file, done) {
     var minFile = file.replace(/\.js$/, '.min.js');
+    var mapFile = minFile + '.map';
+    var mapFileName = mapFile.match(/[^\/]+$/)[0];
     shell.exec(
         'java ' +
             this.java32flags() + ' ' +
             '-jar components/closure-compiler/compiler.jar ' +
             '--compilation_level SIMPLE_OPTIMIZATIONS ' +
             '--language_in ECMASCRIPT5_STRICT ' +
+            '--source_map_format=V3 ' +
+            '--create_source_map ' + mapFile + ' ' +
             '--js ' + file + ' ' +
             '--js_output_file ' + minFile,
       function(code) {
         if (code !== 0) grunt.fail.warn('Error minifying ' + file);
-        grunt.file.write(minFile, this.singleStrict(grunt.file.read(minFile), '\n'));
+
+        // closure creates the source map relative to build/ folder, we need to strip those references
+        grunt.file.write(mapFile, grunt.file.read(mapFile).replace('"file":"build/', '"file":"').
+                                                           replace('"sources":["build/','"sources":["'));
+
+        // move add use strict into the closure + add source map pragma
+        grunt.file.write(minFile, this.sourceMap(mapFileName, this.singleStrict(grunt.file.read(minFile), '\n')));
         grunt.log.ok(file + ' minified into ' + minFile);
         done();
     }.bind(this));