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

tests/lib/mirrors/unused_mirrors4_test.dart

 // Copyright (c) 2013, 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 test.parameter_is_const;
+// 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 get foo => new Bar();
 
-class Class {
-  foo(
-  const /// 01: compile-time error
-  param) {}
+typedef int FunctionTypeDef();
+
+class Bar {
+  final FunctionTypeDef gee;
 }
 
 main() {
-  MethodMirror mm = reflectClass(Class).declarations[#foo];
-  Expect.isFalse(mm.parameters.single.isConst);
+  // Don't do anything.
 }