dart2js: add initial support for lookup-maps

tests/compiler/dart2js/mock_libraries.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.
 
 // Library for creating mock versions of platform and internal libraries.
 
 library mock_libraries;
 
 String buildLibrarySource(
     Map<String, String> elementMap,
@@ skipping 379 matching lines @@
 const Map<String, String> ASYNC_AWAIT_LIBRARY = const <String, String>{
   '_wrapJsFunctionForAsync': '_wrapJsFunctionForAsync(f) {}',
   '_asyncHelper': '_asyncHelper(o, f, c) {}',
 };
 
 const Map<String, String> DEFAULT_MIRRORS_LIBRARY = const <String, String>{
   'Comment': 'class Comment {}',
   'MirrorSystem': 'class MirrorSystem {}',
   'MirrorsUsed': 'class MirrorsUsed {}',
 };
+
+const Map<String, String> DEFAULT_LOOKUP_MAP_LIBRARY = const <String, String>{
+  'LookupMap': r'''
+  class LookupMap<T> {
+    final _key;
+    final _value;
+    final _entries;
+    final _nestedMaps;
+
+    const LookupMap(this._entries, [this._nestedMaps = const []])
+        : _key = null, _value = null;
+
+    const LookupMap.pair(this._key, this._value)
+        : _entries = const [], _nestedMaps = const [];
+  }''',
+};