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

tests/lib/mirrors/unused_mirrors2_test.dart

Index: tests/lib/mirrors/unused_mirrors2_test.dart
diff --git a/tests/lib/mirrors/unused_mirrors2_test.dart b/tests/lib/mirrors/unused_mirrors2_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..f58bdbaa6243f58e8d1a8db8a5f1b29f03a63202
--- /dev/null
+++ b/tests/lib/mirrors/unused_mirrors2_test.dart
@@ -0,0 +1,23 @@
+// Copyright (c) 2014, 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.
+
+// Tests that MirrorsUsed without usage of the static field does not crash
+// dart2js.
+
+library lib;
+
+@MirrorsUsed(targets: const ["lib"])
+import 'dart:mirrors';
+
+Bar foo = new Bar();
+
+typedef int FunctionTypeDef();
+
+class Bar {
+  final FunctionTypeDef gee;
+}
+
+main() {
+  // Don't do anything.
+}