Implement getRange (returning an Iterable).

samples/swarm/swarm_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:collection';
 
 part 'ChangeEvent.dart';
 part 'EventBatch.dart';
@@ skipping 251 matching lines @@
   }
 
   void insertRange(int start, int length, [initialValue = null]) {
     throw new UnimplementedError();
   }
 
   List sublist(int start, [int end]) {
     throw new UnimplementedError();
   }
 
-  List getRange(int start, int length) {
+  Iterable getRange(int start, int end) {
     throw new UnimplementedError();
   }
 
   bool contains(T element) {
     throw new UnimplementedError();
   }
 
   T reduce(T combine(T previousValue, T element)) {
     throw new UnimplementedError();
   }
@@ skipping 52 matching lines @@
     // Only fire on an actual change.
     if (!identical(newValue, _value)) {
       final oldValue = _value;
       _value = newValue;
       recordPropertyUpdate("value", newValue, oldValue);
     }
   }
 
   T _value;
 }