Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/observe/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>';
+}