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/parameter_is_const_test.dart b/tests/lib/mirrors/unused_mirrors2_test.dart
similarity index 50%
copy from tests/lib/mirrors/parameter_is_const_test.dart
copy to tests/lib/mirrors/unused_mirrors2_test.dart
index 45185a9b88d15539aa12dbc1f65b2cc2269e7be3..9ce81af122926d2595bb198d096586a285c2d64f 100644
--- a/tests/lib/mirrors/parameter_is_const_test.dart
+++ b/tests/lib/mirrors/unused_mirrors2_test.dart
@@ -2,19 +2,22 @@
 // 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.
 
+library lib;
+
+@MirrorsUsed(targets: const ["lib"])
 import 'dart:mirrors';
 
-import 'package:expect/expect.dart';
+Bar foo = new Bar();
+
+typedef int FunctionTypeDef();
 
-class Class {
-  foo(
-  const /// 01: compile-time error
-  param) {}
+class Bar {
+  final FunctionTypeDef gee;
 }
 
 main() {
-  MethodMirror mm = reflectClass(Class).declarations[#foo];
-  Expect.isFalse(mm.parameters.single.isConst);
+  // Don't do anything.
 }