dart2js: Use frequency of occurence to sort metadata indices.

pkg/compiler/lib/src/js_emitter/helpers.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.
 
 part of dart2js.js_emitter;
 
 ClassElement computeMixinClass(MixinApplicationElement mixinApplication) {
   ClassElement mixin = mixinApplication.mixin;
   while (mixin.isMixinApplication) {
     mixinApplication = mixin;
     mixin = mixinApplication.mixin;
   }
   return mixin;
 }
+
+class _DeferredOutputUnitHash extends jsAst.DeferredString {
+  String _hash;
+  final OutputUnit _outputUnit;
+
+  _DeferredOutputUnitHash(this._outputUnit);
+
+  void setHash(String hash) {
+    assert(_hash == null);
+    _hash = hash;
+  }
+
+  String get value {
+    assert(_hash != null);
+    return '"$_hash"';
+  }
+
+  String toString() => "HashCode for ${_outputUnit} [$_hash]";
+}