Using 'interface' and 'abstract' for methods should produce error

samples/swarm/UIState.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.
 
 /**
  * The base class for UI state that intends to support browser history.
  */
-class UIState {
+abstract class UIState {
   /**
    * The event listener we hook to the window's "popstate" event.
    * This event is triggered by the back button or by the first page load.
    */
   EventListener _historyTracking;
 
   UIState();
 
   void startHistoryTracking() {
     stopHistoryTracking();
@@ skipping 38 matching lines @@
 
     // TODO(jmesserly): [state] should be an Object, and we should pass it to
     // the state parameter instead of as a #hash URL. Right now we're working
     //  around b/4582542.
     window.history.pushState(null, '${document.title}#$state');
   }
 
   /**
    * Serialize the state to a form suitable for storing in browser history.
    */
-  abstract Map<String, String> toHistory();
+  Map<String, String> toHistory();
 
   /**
    * Load the UI state from the given [values].
    */
-  abstract void loadFromHistory(Map<String, String> values);
+  void loadFromHistory(Map<String, String> values);
 }