Stop importing js_helper.dart explicitly in the core library.

lib/compiler/implementation/lib/interceptors.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.
 
 #library('dart:_interceptors');
 
-#import('coreimpl.dart');
-#import('js_helper.dart');
+#import('dart:coreimpl');
 
 add$1(var receiver, var value) {
   if (isJsArray(receiver)) {
     checkGrowable(receiver, 'add');
     JS('Object', @'#.push(#)', receiver, value);
     return;
   }
   return UNINTERCEPTED(receiver.add(value));
 }
 
@@ skipping 640 matching lines @@
   } else if (receiver is String) {
     return getOrCreateCachedRuntimeType('String');
   } else if (receiver is double) {
     return getOrCreateCachedRuntimeType('double');
   } else if (receiver is List) {
     return getOrCreateCachedRuntimeType('List');
   } else {
     return UNINTERCEPTED(receiver.runtimeType());
   }
 }