Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/observe/src/bind_property.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.bind_property;
+
+import 'dart:async';
+import 'package:observe/observe.dart';
+
+/// Forwards an observable property from one object to another. For example:
+///
+///     class MyModel extends Observable {
+///       StreamSubscription _sub;
+///       MyOtherModel _otherModel;
+///
+///       MyModel() {
+///         ...
+///         _sub = onPropertyChange(_otherModel, #value,
+///             () => notifyPropertyChange(#prop, oldValue, newValue);
+///       }
+///
+///       String get prop => _otherModel.value;
+///       set prop(String value) { _otherModel.value = value; }
+///     }
+///
+/// See also [notifyPropertyChange].
+// TODO(jmesserly): make this an instance method?
+StreamSubscription onPropertyChange(Observable source, Symbol sourceName,
+    void callback()) {
+  return source.changes.listen((records) {
+    for (var record in records) {
+      if (record is PropertyChangeRecord &&
+          (record as PropertyChangeRecord).name == sourceName) {
+        callback();
+        break;
+      }
+    }
+  });
+}