Remove support for interface in dart2js.

tests/compiler/dart2js_native/native_class_is_check1_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.
 
 import "package:expect/expect.dart";
 
 // Test for correct simple is-checks on hidden native classes.
 
-interface I {
+abstract class I {
   I read();
   write(I x);
 }
 
 // Native implementation.
 
 class A implements I native "A" {
   // The native class accepts only other native instances.
   A read() native;
   write(A x) native;
@@ skipping 18 matching lines @@
   var ob = new Object();
 
   Expect.isFalse(ob is I);
   Expect.isFalse(ob is A);
   Expect.isFalse(ob is B);
 
   Expect.isTrue(a1 is I);
   Expect.isTrue(a1 is A);
   Expect.isTrue(a1 is !B);
 }