Don't hide interceptors in mirrors.

tests/utils/dummy_compiler_test.dart

 // Copyright (c) 2012, 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.
 // VMOptions=
 // VMOptions=--print-object-histogram
 
 // Smoke test of the dart2js compiler API.
 library dummy_compiler;
 
 import 'dart:async';
 import "package:async_helper/async_helper.dart";
 
 import '../../sdk/lib/_internal/compiler/compiler.dart';
 
 Future<String> provider(Uri uri) {
   String source;
   if (uri.scheme == "main") {
     source = "main() {}";
   } else if (uri.scheme == "lib") {
     if (uri.path.endsWith("/core.dart")) {
       source = """
 library core;
 class Object {
   Object();
+  // Note: JSNull below must reimplement all members.
   operator==(other) {}
   get hashCode => throw 'Object.hashCode not implemented.';
 }
 class Type {}
 class bool {}
 class num {}
 class int {}
 class double{}
 class String{}
 class Function{}
@@ skipping 42 matching lines @@
   var toString;
   get length => 0;
 }
 class JSFunction {}
 class JSInt {}
 class JSPositiveInt {}
 class JSUInt31 {}
 class JSUInt32 {}
 class JSDouble {}
 class JSNumber {}
-class JSNull {}
+class JSNull {
+  bool operator ==(other) => identical(null, other);
+  int get hashCode => 0;
+}
 class JSBool {}
 getInterceptor(o){}
 getDispatchProperty(o) {}
 setDispatchProperty(o, v) {}
 var mapTypeToInterceptor;""";
     } else if (uri.path.endsWith('js_helper.dart')) {
       source = 'library jshelper; class JSInvocationMirror {} '
                'class ConstantMap {} class TypeImpl {} '
                'createRuntimeType(String name) => null;';
     } else if (uri.path.endsWith('isolate_helper.dart')) {
@@ skipping 23 matching lines @@
               new Uri(scheme: 'package', path: '/'),
               provider, handler);
   result.then((String code) {
     if (code == null) {
       throw 'Compilation failed';
     }
   }, onError: (e) {
       throw 'Compilation failed';
   }).then(asyncSuccess);
 }