Remove code supporting incremental compilation in dart2js

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;
-}