Allow using native JS []= on typed data lists.

tests/compiler/dart2js/mock_compiler.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 mock_compiler;
 
 import "package:expect/expect.dart";
 import 'dart:collection';
 import 'dart:uri';
 
@@ skipping 63 matching lines @@
 
 const String DEFAULT_INTERCEPTORSLIB = r'''
   class Interceptor {
     toString() {}
     bool operator==(other) => identical(this, other);
     noSuchMethod(im) { throw im; }
   }
   abstract class JSIndexable {
     get length;
   }
+  abstract class JSMutableIndexable extends JSIndexable {}
   class JSArray extends Interceptor implements List, JSIndexable {
     var length;
     operator[](index) {}
     operator[]=(index, value) {}
     var add;
   }
-  class JSMutableArray extends JSArray {}
+  class JSMutableArray extends JSArray implements JSMutableIndexable {}
   class JSFixedArray extends JSMutableArray {}
   class JSExtendableArray extends JSMutableArray {}
   class JSString extends Interceptor implements String, JSIndexable {
     var length;
     operator[](index) {}
     toString() {}
   }
   class JSNumber extends Interceptor implements num {
     // All these methods return a number to please type inferencing.
     operator-() => (this is JSInt) ? 42 : 42.0;
@@ skipping 299 matching lines @@
   }
 }
 
 class MockDeferredLoadTask extends DeferredLoadTask {
   MockDeferredLoadTask(Compiler compiler) : super(compiler);
 
   void registerMainApp(LibraryElement mainApp) {
     // Do nothing.
   }
 }