Various caches for incremental compilation.

dart/sdk/lib/_internal/compiler/implementation/cache_strategy.dart

Index: dart/sdk/lib/_internal/compiler/implementation/cache_strategy.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/cache_strategy.dart b/dart/sdk/lib/_internal/compiler/implementation/cache_strategy.dart
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+// 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 {
+  Map newMap() => new HashMap.identity();
+
+  Set newSet() => new HashSet.identity();
+}