Update mock of dart:core in polymer compiler (fixes dartbug.com/19560)

pkg/polymer/lib/src/build/script_compactor.dart

 // Copyright (c) 2013, 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.
 
 /// Transfomer that combines multiple dart script tags into a single one.
 library polymer.src.build.script_compactor;
 
 import 'dart:async';
 import 'dart:convert';
 
@@ skipping 57 matching lines @@
             class int extends num {}
             class double extends num {}
             class DateTime extends Object {}
             class Null extends Object {}
 
             class Deprecated extends Object {
               final String expires;
               const Deprecated(this.expires);
             }
             const Object deprecated = const Deprecated("next release");
+            class _Override { const _Override(); }
+            const Object override = const _Override();
+            class _Proxy { const _Proxy(); }
+            const Object proxy = const _Proxy();
 
             class List<V> extends Object {}
             class Map<K, V> extends Object {}
             ''',
         'dart:html': '''
             library dart.html;
             class HtmlElement {}
             ''',
       });
 
@@ skipping 678 matching lines @@
   for (var c in combinators) {
     if (c is ShowElementCombinator) {
       var show = c.shownNames.toSet();
       elements.retainWhere((e) => show.contains(e.displayName));
     } else if (c is HideElementCombinator) {
       var hide = c.hiddenNames.toSet();
       elements.removeWhere((e) => hide.contains(e.displayName));
     }
   }
 }