Don't cache non-Interceptor interceptors.

dart/tests/lib/mirrors/intercepted_cache_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.
+
+// This is a test for a problem in how dart2js cached InstanceMirror.invoke,
+// etc. The test is using getField, as invoke, setField, and getField all share
+// the same caching.
+
+import 'dart:mirrors';
+
+import 'package:expect/expect.dart';
+
+
+class Foo {
+  Foo(this.length);
+  int length;
+}
+
+main() {
+  Expect.equals(
+      1, reflect(new Foo(1)).getField(const Symbol('length')).reflectee);
+  Expect.equals(
+      2, reflect(new Foo(2)).getField(const Symbol('length')).reflectee);
+  Expect.equals(0, reflect([]).getField(const Symbol('length')).reflectee);
+}