dart2js: Use frequency of occurence to sort metadata indices.

pkg/compiler/lib/src/js_emitter/js_emitter.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.
 
 library dart2js.js_emitter;
 
 import 'dart:convert';
+import 'dart:collection' show LinkedHashMap;
 
 import '../common.dart';
 
 import '../constants/expressions.dart';
 import '../constants/values.dart';
 
 import '../closure.dart' show
     ClosureClassElement,
     ClosureClassMap,
     ClosureFieldElement,
@@ skipping 13 matching lines @@
     MethodElement;
 
 import '../hash/sha1.dart' show Hasher;
 
 import '../helpers/helpers.dart';  // Included for debug helpers.
 
 import '../js/js.dart' as jsAst;
 import '../js/js.dart' show
     js;
 
+import 'package:js_ast/src/precedence.dart' as js_precedence;
+
 import '../js_backend/js_backend.dart' show
     CheckedModeHelper,
     ConstantEmitter,
     CustomElementsAnalysis,
     JavaScriptBackend,
     JavaScriptConstantCompiler,
     Namer,
     RuntimeTypes,
     Substitution,
     TypeCheck,
@@ skipping 54 matching lines @@
 
 part 'old_emitter/class_builder.dart';
 part 'old_emitter/class_emitter.dart';
 part 'old_emitter/code_emitter_helper.dart';
 part 'old_emitter/container_builder.dart';
 part 'old_emitter/declarations.dart';
 part 'old_emitter/emitter.dart';
 part 'old_emitter/interceptor_emitter.dart';
 part 'old_emitter/nsm_emitter.dart';
 part 'old_emitter/setup_program_builder.dart';