Avoid prototype properties in JS-Object caches.

tests/lib/mirrors/invocation_cache_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.
 
-// Test that classes referenced from a signature of a tear-off closure
-// are emitted.
-
-@MirrorsUsed(targets: 'C')
+@MirrorsUsed(targets: const ["A"])
 import 'dart:mirrors';
 
 import 'package:expect/expect.dart';
 
-class A {}
-
-class C {
-  A foo() {}
+class A {
+  toString() => "A";
 }
 
 main() {
-  Expect.isFalse(reflect(new C().foo).function.returnType.toString()
-      .contains('dynamic'));
+  // The invocation cache must not find the 'toString' from JavaScript's
+  // Object.prototype.
+  var toString = reflect(new A()).getField(#toString).reflectee;
+  Expect.equals("A", Function.apply(toString, [], {}));
 }