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

samples/ui_lib/touch/Momentum.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.
 
 /**
  * Implementations can be used to simulate the deceleration of an element within
  * a certain region. To use this behavior you need to provide an initial
  * velocity that is meant to represent the gesture that is initiating this
  * deceleration. You also provide the bounds of the region that the element
  * exists in, and the current offset of the element within that region. The
  * transitions will have the element decelerate to rest, or stretch past the
  * offset boundaries and then come to rest.
  *
  * This is primarily designed to solve the problem of slow scrolling in mobile
  * safari. You can use this along with the [Scroller] behavior to make a
  * scrollable area scroll the same way it would in a native application.
  *
  * Implementations of this interface do not maintain any references to HTML
  * elements, and therefore cannot do any redrawing of elements. They only
  * calculates where the element should be on an interval. It is the delegate's
  * responsibility to redraw the element when the onDecelerate callback is
  * invoked. It is recommended that you move the element with a hardware
  * accelerated method such as using 'translate3d' on the element's
  * -webkit-transform style property.
  */
-interface Momentum default TimeoutMomentum {
+abstract class Momentum {
 
-  Momentum(MomentumDelegate delegate, [num defaultDecelerationFactor]);
+  factory Momentum(MomentumDelegate delegate, [num defaultDecelerationFactor]) = TimeoutMomentum;
 
   bool get decelerating;
 
   num get decelerationFactor;
 
  /**
   * Transition end handler. This function must be invoked after any transition
   * that occurred as a result of a call to the delegate's onDecelerate callback.
   */
   void onTransitionEnd();
@@ skipping 506 matching lines @@
 
   Coordinate get destination {
     if (!_moves.isEmpty) {
       final lastMove = _moves.last;
       return new Coordinate(lastMove.x, lastMove.y);
     } else {
       return null;
     }
   }
 }