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

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

Index: pkg/polymer/lib/src/build/script_compactor.dart
diff --git a/pkg/polymer/lib/src/build/script_compactor.dart b/pkg/polymer/lib/src/build/script_compactor.dart
index 8127edaa5cb5c89598b1d703d1c7747b50e21e63..e365aab9eb4295adf649211e83c2671cc43ea873 100644
--- a/pkg/polymer/lib/src/build/script_compactor.dart
+++ b/pkg/polymer/lib/src/build/script_compactor.dart
@@ -75,6 +75,10 @@ class ScriptCompactor extends Transformer {
               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 {}