Move some core classes to collection library.

tests/compiler/dart2js/mock_compiler.dart

 // Copyright (c) 2012, 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 mock_compiler;
 
+import 'dart:collection';
 import 'dart:uri';
 
 import '../../../sdk/lib/_internal/compiler/compiler.dart' as api;
 import '../../../sdk/lib/_internal/compiler/implementation/elements/elements.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/resolution/resolution.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/source_file.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/tree/tree.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/util/util.dart';
 import 'parser_helper.dart';
 
@@ skipping 304 matching lines @@
   operator []=(Node node, Element element) {
     map[node] = element;
   }
 
   operator [](Node node) => map[node];
 
   void remove(Node node) {
     map.remove(node);
   }
 }