Fix JS-backend when MirrorsUsed target a static field, but there is no mirrors usage.

tests/lib/mirrors/unused_mirrors_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file

[Johnni Winther, 2014/01/30 11:47:18]

This test looks identical to unused_mirrors4_test.

[floitsch, 2014/01/30 12:27:23]

4-test uses a getter.

 // 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.
 
+// Tests that MirrorsUsed without usage of the static field does not crash
+// dart2js.
+
+@MirrorsUsed(targets: const ["foo"])
 import 'dart:mirrors';
 
-import 'package:expect/expect.dart';
+Bar foo = new Bar();
 
-class Foo {
+typedef int FunctionTypeDef();
+
+class Bar {
+  final FunctionTypeDef gee;
 }
 
 main() {
-  Expect.stringEquals('Foo', '${new Foo().runtimeType}');
-  Expect.stringEquals('foo', MirrorSystem.getName(new Symbol('foo')));
+  // Don't do anything.
 }