Avoid prototype properties in JS-Object caches.

tests/lib/mirrors/get_field_cache_test.dart

Index: tests/lib/mirrors/get_field_cache_test.dart
diff --git a/tests/lib/mirrors/get_field_cache_test.dart b/tests/lib/mirrors/get_field_cache_test.dart
new file mode 100644
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--- /dev/null
+++ b/tests/lib/mirrors/get_field_cache_test.dart
@@ -0,0 +1,36 @@
+// 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.
+
+@MirrorsUsed(targets: const ["A", "B"])
+import 'dart:mirrors';
+
+import 'package:expect/expect.dart';
+
+class A {
+  toString() => "A";
+}
+
+class B {
+  int x = 99;
+  toString() => "B";
+}
+
+void main() {
+  var a = new A();
+  var am = reflect(a);
+  for (int i = 0; i < 10; i++) {
+    // Adds a probe function on the symbol.
+    am.getField(#toString);
+  }
+  var b = new B();
+  var bm = reflect(b);
+  for (int i = 0; i < 10; i++) {
+    // Adds a field-cache on the mirror.
+    bm.getField(#x);
+  }
+  // There is a cache now, but the cache should not contain 'toString' from
+  // JavaScript's Object.prototype.
+  var toString = bm.getField(#toString).reflectee;
+  Expect.equals("B", toString());
+}