Fix some errors around []= in core types.

pkg/dev_compiler/tool/input_sdk/private/js_helper.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_helper;
 
 import 'dart:collection';
 
 import 'dart:_foreign_helper' show
     JS,
@@ skipping 750 matching lines @@
 class JSName {
   final String name;
   const JSName(this.name);
 }
 
 /**
  * Special interface recognized by the compiler and implemented by DOM
  * objects that support integer indexing. This interface is not
  * visible to anyone, and is only injected into special libraries.
  */
-abstract class JavaScriptIndexingBehavior extends JSMutableIndexable {
+abstract class JavaScriptIndexingBehavior {
 }
 
 // TODO(lrn): These exceptions should be implemented in core.
 // When they are, remove the 'Implementation' here.
 
 /** Thrown by type assertions that fail. */
 class TypeErrorImplementation extends Error implements TypeError {
   final String message;
 
   /**
@@ skipping 112 matching lines @@
   // we have no way of telling the compiler yet, so it will generate an extra
   // layer of indirection that wraps the SyncIterator.
   _jsIterator() => JS('', '#(...#)', _generator, _args);
 
   Iterator<E> get iterator => new SyncIterator<E>(_jsIterator());
 }
 
 class BooleanConversionAssertionError extends AssertionError {
   toString() => 'Failed assertion: boolean expression must not be null';
 }