Remove support for interface in dart2js.

tests/compiler/dart2js_native/downcast_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.
 
 // Test for downcasts on native classes.
 
 import "package:expect/expect.dart";
 
-interface J {
+abstract class J {
 }
 
-interface I extends J {
+abstract class I extends J {
   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 48 matching lines @@
   Expect.equals(b1, b1 as I);
   Expect.equals(b1, b1 as A);
   Expect.equals(b1, b1 as B);
 
   Expect.equals(a1, a1 as J);
   Expect.equals(a1, a1 as I);
   Expect.equals(a1, a1 as A);
   Expect.throws(() => a1 as B);
   Expect.throws(() => a1 as C);
 }