Changed interfaces to abstract classes where possible in {lib,samples,pkg}

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
@@ skipping 52 matching lines @@
 
   /** null if no transition is in progress. */
   Coordinate get destination;
 }
 
 /**
  * Momentum Delegate interface.
  * You are required to implement this interface in order to use the
  * Momentum behavior.
  */
-interface MomentumDelegate {
+abstract class MomentumDelegate {
   /**
    * Callback for a deceleration step. The delegate is responsible for redrawing
    * the element in its new position specified in px.
    */
   void onDecelerate(num x, num y);
 
   /**
    * Callback for end of deceleration.
    */
   void onDecelerationEnd();
@@ skipping 461 matching lines @@
 
   Coordinate get destination {
     if (!_moves.isEmpty()) {
       final lastMove = _moves.last();
       return new Coordinate(lastMove.x, lastMove.y);
     } else {
       return null;
     }
   }
 }