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

samples/ui_lib/layout/SizingFunctions.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.
 
 // This file has classes representing the grid sizing functions
 
 /**
  * Represents the sizing function used for the min or max of a row or column.
  */
 // TODO(jmesserly): rename to GridSizing, or make internal
@@ skipping 104 matching lines @@
       max = const MaxContentSizing();
 
   TrackSizing(this.min, this.max) {}
 
   // TODO(jmesserly): this is only needed because FixedSizing is mutable
   TrackSizing clone() => new TrackSizing(min.clone(), max.clone());
 }
 
 /** Represents a GridTrack breadth property. */
 // TODO(jmesserly): these classes could be replaced with reflection/mirrors
-interface _BreadthAccumulator {
+abstract class _BreadthAccumulator {
   void setSize(GridTrack t, num value);
   num getSize(GridTrack t);
 
   SizingFunction getSizingFunction(GridTrack t);
 }
 
 class _UsedBreadthAccumulator implements _BreadthAccumulator {
   const _UsedBreadthAccumulator();
 
   void setSize(GridTrack t, num value) { t.usedBreadth = value; }
   num getSize(GridTrack t) => t.usedBreadth;
 
   SizingFunction getSizingFunction(GridTrack t) => t.minSizing;
 }
 
 class _MaxBreadthAccumulator implements _BreadthAccumulator {
   const _MaxBreadthAccumulator();
 
   void setSize(GridTrack t, num value) { t.maxBreadth = value; }
   num getSize(GridTrack t) => t.maxBreadth;
 
   SizingFunction getSizingFunction(GridTrack t) => t.maxSizing;
 }