dart2js: add initial support for lookup-maps

tests/compiler/dart2js_extra/lookup_map/dead_entry_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{}
+const map = const LookupMap(const [
+    A, "the-text-for-A",
+    B, "the-text-for-B",
+]);
 
-class T {
-  A foo(int x) {}
+main() {
+  Expect.equals(map[A], "the-text-for-A");
 }
-
-class A{}
-
-typedef A F(int x);
-
-use(x) => x;
-
-runTest() {
-  use(new A());
-  Expect.isTrue(new T().foo is F);
-}