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

tests/lib/mirrors/unused_mirrors3_test.dart

+// 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 ["C"])
+import 'dart:mirrors';
+
+class C {
+  static Bar foo = new Bar();
+}
+
+typedef int FunctionTypeDef();
+
+class Bar {
+  final FunctionTypeDef gee;
+}
+
+main() {
+  // Don't do anything.
+}