Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/observe/src/change_record.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.change_record;
+
+import 'package:observe/observe.dart';
+
+
+/// Records a change to an [Observable].
+// TODO(jmesserly): remove this type
+abstract class ChangeRecord {}
+
+/// A change record to a field of an observable object.
+class PropertyChangeRecord<T> extends ChangeRecord {
+  /// The object that changed.
+  final object;
+
+  /// The name of the property that changed.
+  final Symbol name;
+
+  /// The previous value of the property.
+  final T oldValue;
+
+  /// The new value of the property.
+  final T newValue;
+
+  PropertyChangeRecord(this.object, this.name, this.oldValue, this.newValue);
+
+  String toString() =>
+      '#<PropertyChangeRecord $name from: $oldValue to: $newValue>';
+}