Convert legacy 'native code' to JS-forms in dart:html

lib/html/templates/html/dart2js/factoryprovider_Blob.darttemplate

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class $FACTORYPROVIDER {
   static Blob createBlob([List blobParts = null, String type, String endings]) {
     // TODO: validate that blobParts is a JS Array and convert if not.
     // TODO: any coercions on the elements of blobParts, e.g. coerce a typed
     // array to ArrayBuffer if it is a total view.
     if (type == null && endings == null) {
       return _create_1(blobParts);
     }
     var bag = _create_bag();
     if (type != null) _bag_set(bag, 'type', type);
     if (endings != null) _bag_set(bag, 'endings', endings);
     return _create_2(blobParts, bag);
   }
 
-  static _create_1(parts) native 'return new Blob(parts);';
-  static _create_2(parts, bag) native 'return new Blob(parts, bag);';
+  static _create_1(parts) => JS('Blob', 'new Blob(#)', parts);
+  static _create_2(parts, bag) => JS('Blob', 'new Blob(#, #)', parts, bag);
 
-  static _create_bag() native 'return {}';
-  static _bag_set(bag, key, value) native 'bag[key] = value;';
+  static _create_bag() => JS('var', '{}');
+  static _bag_set(bag, key, value) { JS('void', '#[#] = #', bag, key, value); }
 }