Don't cache non-Interceptor interceptors.

dart/sdk/lib/_internal/lib/js_mirrors.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.
 
 library dart._js_mirrors;
 
 import 'dart:async';
 import 'dart:collection' show UnmodifiableListView;
 import 'dart:mirrors';
 
@@ skipping 487 matching lines @@
                          int type,
                          String reflectiveName,
                          List arguments) {
     String cacheName = Primitives.mirrorInvokeCacheName;
     var cache = JS('', r'#.constructor[#]', reflectee, cacheName);
     if (cache == null) {
       cache = JsCache.allocate();
       JS('void', r'#.constructor[#] = #', reflectee, cacheName, cache);
     }
     var cacheEntry = JsCache.fetch(cache, reflectiveName);
-    cacheEntry = null; // Work around bug in caching.
     var result;
     Invocation invocation;
     if (cacheEntry == null) {
       disableTreeShaking();
       String mangledName = reflectiveNames[reflectiveName];
       // TODO(ahe): Get the argument names.
       List<String> argumentNames = [];
       // TODO(ahe): We don't need to create an invocation mirror here. The
       // logic from JSInvocationMirror.getCachedInvocation could easily be
       // inlined here.
@@ skipping 834 matching lines @@
   void operator []=(K key, V value) => _throw();
 
   V putIfAbsent(K key, V ifAbsent()) { _throw(); }
 
   void addAll(Map<K, V> other) => _throw();
 
   V remove(K key) { _throw(); }
 
   void clear() => _throw();
 }