Remove support for interface in dart2js.

tests/compiler/dart2js_native/native_field_rename_2_frog_test.dart

 // Copyright (c) 2011, 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.
 
 // A native method prevents other members from having that name, including
 // fields.  However, native fields keep their name.  The implication: a getter
 // for the field must be based on the field's name, not the field's jsname.
 
 import "package:expect/expect.dart";
 import 'dart:_js_helper' show JSName;
 
-interface I {
+abstract class I {
   int key;
 }
 
 class A implements I native "A" {
   int key;                    //  jsname is 'key'
   int getKey() => key;
 }
 
 class B implements I {
   int key;                    //  jsname is not 'key'
@@ skipping 66 matching lines @@
   Expect.equals(222, b.getKey());
 }
 
 main() {
   setup();
 
   testTyped();
   testPartial();
   testDynamic();
 }