Update charted to 0.4.8 and roll

packages/charted/lib/charts/data_transformers/aggregation_item.dart

 //
 // Copyright 2014 Google Inc. All rights reserved.
 //
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file or at
 // https://developers.google.com/open-source/licenses/bsd
 //
 
 part of charted.charts;
 
 /// AggregationItem is created by [AggregationModel] to make access to facts
 /// observable. Users must use AggregationItem.isValid before trying to access
 /// the aggregations.
-abstract class AggregationItem extends ChangeNotifier {
+abstract class AggregationItem extends Observable {
   /// List of dimension fields in effect
   List<String> dimensions;
 
   /// Check if this entity is valid.
   /// Currently the only case where an entity becomes invalid
   /// is when a groupBy is called on the model.
   bool get isValid;
 
   /// Fetch the fact from AggregationModel and return it
   /// Currently takes keys in the form of "sum(spend)", where sum is
@@ skipping 10 matching lines @@
   bool containsKey(String key);
 
   /// List of valid field names for this entity.
   /// It's the combined list of accessors for individual items, items in
   /// the next dimension and all possible facts defined on the view.
   Iterable<String> get fieldNames;
 }
 
 /// Implementation of AggregationItem
 /// Instances of _AggregationItemImpl are created only by AggregationModel
-class _AggregationItemImpl extends ChangeNotifier implements AggregationItem {
+class _AggregationItemImpl extends Observable implements AggregationItem {
   static final List<String> derivedAggregationTypes = ['count', 'avg'];
 
   AggregationModel model;
   List<String> dimensions;
 
   String _key;
 
   int _factsOffset;
 
   /// Currently entities are created only when they have valid aggregations
@@ skipping 177 matching lines @@
     return (_current < _endOfRows);
   }
 
   get current {
     if (_current == null || _counter > _count) {
       return null;
     }
     return model._rows[model._sorted[_current]];
   }
 }