Move dart2js from sdk/lib/_internal/compiler to pkg/compiler

pkg/compiler/lib/src/cache_strategy.dart

 // Copyright (c) 2014, 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.
 
 import 'dart:collection' show
     HashMap,
     HashSet;
 
 /**
  * Helper class for allocating sets and maps appropriate for caching objects
  * that can be assumed to be canonicalized.
  *
  * When compiling dart2js to JavaScript, profiling reveals that identity maps
  * and sets have superior performance.  However, we know that [Object.hashCode]
  * is slow on the Dart VM.  This class is meant to encapsulate the decision
  * about which data structure is best, and we anticipate specific subclasses
  * for JavaScript and Dart VM in the future.
  */
 class CacheStrategy {
   final bool hasIncrementalSupport;
 
   CacheStrategy(this.hasIncrementalSupport);
 
   Map newMap() => hasIncrementalSupport ? new HashMap.identity() : null;
 
   Set newSet() => hasIncrementalSupport ? new HashSet.identity() : null;
 }