Change interfaces to abstract classes in dart2js compiler.

lib/compiler/implementation/lib/mockimpl.dart

Index: lib/compiler/implementation/lib/mockimpl.dart
diff --git a/lib/compiler/implementation/lib/mockimpl.dart b/lib/compiler/implementation/lib/mockimpl.dart
deleted file mode 100644
index 40b863a0641d8b0156c8127c56318771b17ffa37..0000000000000000000000000000000000000000
--- a/lib/compiler/implementation/lib/mockimpl.dart
+++ /dev/null
@@ -1,13 +0,0 @@
-// 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.
-
-// Mocks of classes and interfaces that Leg cannot read directly.
-
-// TODO(ahe): Remove this file.
-
-class ReceivePortFactory {
-  factory ReceivePort() {
-    throw 'factory ReceivePort is not implemented';
-  }
-}