Remove support for interface in dart2js.

tests/compiler/dart2js_native/native_window1_frog_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.
 
 import "package:expect/expect.dart";
 
-interface Window {
+abstract class Window {
   final int document;
 }
 
 // Defining this global object makes Frog eager on optimizing
 // call sites where the receiver is typed 'Window'.
 class _DOMWindowJs implements Window native "@*DOMWindow" {
   final int document;
 }
 
 class Win implements Window {
 }
 
 main() {
   // By typing this variable to 'Window', Frog will optimize calls on
   // it.
   Window win = new Win();
   Expect.throws(() => win.document,
                 (e) => e is NoSuchMethodError);
 }