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

samples/ui_lib/touch/ScrollWatcher.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.
 
-interface ScrollListener {
+abstract class ScrollListener {
   /**
    * The callback invoked for a scroll event.
    * [decelerating] specifies whether or not the content is moving due
    * to deceleration. It should be false if the content is moving because the
    * user is dragging the content.
    */
   void onScrollerMoved(double scrollX, double scrollY, bool decelerating);
 }
 
 /**
@@ skipping 38 matching lines @@
 
   /**
    * This callback is invoked any time the scroller content offset changes.
    */
   void _onContentMoved(Event e) {
     num scrollX = _scroller.getHorizontalOffset();
     num scrollY = _scroller.getVerticalOffset();
     _dispatchScroll(scrollX, scrollY);
   }
 }