dart2js: Emit libraries that are only used for typedefs.

tests/lib/mirrors/typedef_library_test.dart

Index: tests/lib/mirrors/typedef_library_test.dart
diff --git a/tests/lib/mirrors/typedef_library_test.dart b/tests/lib/mirrors/typedef_library_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..6549797a49a0fe338b8a0acb0c5d643ef49a896e
--- /dev/null
+++ b/tests/lib/mirrors/typedef_library_test.dart
@@ -0,0 +1,17 @@
+// 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.
+
+library foo;
+
+@MirrorsUsed(targets: const ["foo", "bar"])
+import 'dart:mirrors';
+import 'typedef_library.dart';
+
+import 'package:expect/expect.dart';
+
+main() {
+  var barLibrary = currentMirrorSystem().findLibrary(new Symbol("bar"));
+  var gTypedef = barLibrary.declarations[new Symbol("G")];
+  Expect.equals("G", MirrorSystem.getName(gTypedef.simpleName));
+}