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

samples/ui_lib/observable/observable.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.
 
 #library('observable');
 
 #import('dart:coreimpl');
 
 #source('ChangeEvent.dart');
 #source('EventBatch.dart');
 
 /**
  * An object whose changes are tracked and who can issue events notifying how it
  * has been changed.
  */
-interface Observable {
+abstract class Observable {
   /** Returns a globally unique identifier for the object. */
   // TODO(sigmund): remove once dart supports maps with arbitrary keys.
-  final int uid;
+  int get uid;
 
   /** Listeners on this model. */
-  final List<ChangeListener> listeners;
+  List<ChangeListener> get listeners;
 
   /** The parent observable to notify when this child is changed. */
-  final Observable parent;
+  Observable get parent;
 
   /**
    * Adds a listener for changes on this observable instance. Returns whether
    * the listener was added successfully.
    */
   bool addChangeListener(ChangeListener listener);
 
   /**
    * Removes a listener for changes on this observable instance. Returns whether
    * the listener was removed successfully.
@@ skipping 254 matching lines @@
     //              equality check should be done?
     if (newValue !== _value) {
       final oldValue = _value;
       _value = newValue;
       recordPropertyUpdate("value", newValue, oldValue);
     }
   }
 
   T _value;
 }