dart2js: add initial support for lookup-maps

tests/compiler/dart2js_extra/lookup_map/discovered_code_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{ A(B x);}
+class B{}
+class C{}
+class D{}
+class E{}
+createA() => new A(map[B][1]());
+createB() => new B();
+const map = const LookupMap(const [
+    A, const ["the-text-for-A", createA],
+    B, const ["the-text-for-B", createB],
+    C, const ["the-text-for-C"],
+]);
+
+main() {
+  Expect.isTrue(map[A][1]() is A);
+}