Update charted to 0.4.8 and roll

packages/observe/lib/src/observable_box.dart

-// Copyright (c) 2013, 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 observe.src.observable_box;
-
-import 'package:observe/observe.dart';
-
-// TODO(jmesserly): should the property name be configurable?
-// That would be more convenient.
-/// An observable box that holds a value. Use this if you want to store a single
-/// value. For other cases, it is better to use [ObservableList],
-/// [ObservableMap], or a custom [Observable] implementation based on
-/// [Observable]. The property name for changes is "value".
-class ObservableBox<T> extends ChangeNotifier {
-  T _value;
-
-  ObservableBox([T initialValue]) : _value = initialValue;
-
-  @reflectable T get value => _value;
-
-  @reflectable void set value(T newValue) {
-    _value = notifyPropertyChange(#value, _value, newValue);
-  }
-
-  String toString() => '#<$runtimeType value: $value>';
-}