Implement @patch annotation for patch class members

runtime/lib/resource_patch.dart

 // Copyright (c) 2015, 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.
 
 @patch class Resource {
-  /* @patch */ const factory Resource(String uri) = _Resource;
+  @patch const factory Resource(String uri) = _Resource;
 }
 
 class _Resource implements Resource {
   final String _location;
 
   const _Resource(this._location);
 
   Uri get uri => Uri.base.resolve(_location);
 
   Stream<List<int>> openRead() {
@@ skipping 42 matching lines @@
       var str = encoding.decode(bytes);
       completer.complete(str);
     },
     onError: (e, s) {
       completer.completeError(e,s);
     });
 
     return completer.future;
   }
 }