Change interfaces to abstract classes in dart2js compiler.

lib/compiler/implementation/lib/isolate_patch.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.
 
 // Patch file for the dart:isolate library.
 
 #import("dart:uri");
 
 /**
  * Called by the compiler to support switching
@@ skipping 346 matching lines @@
   String message;
 
   _IsolateEvent(this.isolate, this.fn, this.message);
 
   void process() {
     isolate.eval(fn);
   }
 }
 
 /** An interface for a stub used to interact with a manager. */
-interface _ManagerStub {
+abstract class _ManagerStub {
   get id;
   void set id(int i);
   void set onmessage(Function f);
   void postMessage(msg);
   void terminate();
 }
 
 /** A stub for interacting with the main manager. */
 class _MainManagerStub implements _ManagerStub {
   get id => 0;
@@ skipping 883 matching lines @@
       _window.clearTimeout(_handle);
     } else {
       _window.clearInterval(_handle);
     }
   }
 }
 
 Timer _timerFactory(int millis, void callback(Timer timer), bool repeating) =>
   repeating ? new _Timer.repeating(millis, callback)
             : new _Timer(millis, callback);