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

samples/ui_lib/layout/ViewLayout.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 interface that the layout algorithms use to talk to the view. */
-interface Positionable {
+abstract class Positionable {
   ViewLayout get layout();
 
   /** Gets our custom CSS properties, as provided by the CSS preprocessor. */
   Map<String, String> get customStyle();
 
   /** Gets the root DOM used for layout. */
   Element get node();
 
   /** Gets the collection of child views. */
   Collection<Positionable> get childViews();
@@ skipping 206 matching lines @@
       // For an unset max-content size, use the actual size
       return size;
     }
     if (style.endsWith('%')) {
       num percent = Math.parseDouble(style.substring(0, style.length - 1));
       return ((percent / 100) * parentSize).toInt();
     }
     return _toPixels(style);
   }
 }