dart2js: add initial support for lookup-maps

tests/compiler/dart2js_extra/lookup_map/reachable_data_test.dart

+// Copyright (c) 2015, 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 'package:lookup_map/lookup_map.dart';
+import 'package:expect/expect.dart';
+
+class A{}
+class B{}
+class C{}
+class D{}
+class E{}
+const map = const LookupMap(const [
+    A, const ["the-text-for-A", B],
+    B, const ["the-text-for-B", C],
+    C, const ["the-text-for-C"],
+    D, const ["the-text-for-D", E],
+    E, const ["the-text-for-E"],
+]);
+main() {
+  Expect.equals(map[map[A][1]][0], 'the-text-for-B');
+}