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

tests/lib/mirrors/unused_mirrors_test.dart

Index: tests/lib/mirrors/unused_mirrors_test.dart
diff --git a/tests/lib/mirrors/unmangled_type_test.dart b/tests/lib/mirrors/unused_mirrors_test.dart
similarity index 51%
copy from tests/lib/mirrors/unmangled_type_test.dart
copy to tests/lib/mirrors/unused_mirrors_test.dart
index dbc6b62437908677c6fb6c1dab670b8d0dd7d2b6..fcbc08aa56898ea761d80d932707acead9bb2245 100644
--- a/tests/lib/mirrors/unmangled_type_test.dart
+++ b/tests/lib/mirrors/unused_mirrors_test.dart
@@ -2,14 +2,20 @@
 // 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();
+
+typedef int FunctionTypeDef();
 
-class Foo {
+class Bar {
+  final FunctionTypeDef gee;
 }
 
 main() {
-  Expect.stringEquals('Foo', '${new Foo().runtimeType}');
-  Expect.stringEquals('foo', MirrorSystem.getName(new Symbol('foo')));
+  // Don't do anything.
 }