Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/observe/src/bindable.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.bindable;
+
+/// An object that can be data bound.
+// Normally this is used with 'package:template_binding'.
+// TODO(jmesserly): Node.bind polyfill calls this "observable"
+abstract class Bindable {
+  // Dart note: changed setValue to be "set value" and discardChanges() to
+  // be "get value".
+
+  /// Initiates observation and returns the initial value.
+  /// The callback will be called with the updated [value].
+  ///
+  /// Some subtypes may chose to provide additional arguments, such as
+  /// [PathObserver] providing the old value as the second argument.
+  /// However, they must support callbacks with as few as 0 or 1 argument.
+  /// This can be implemented by performing an "is" type test on the callback.
+  open(callback);
+
+  /// Stops future notifications and frees the reference to the callback passed
+  /// to [open], so its memory can be collected even if this Bindable is alive.
+  void close();
+
+  /// Gets the current value of the bindings.
+  /// Note: once the value of a [Bindable] is fetched, the callback passed to
+  /// [open] should not be called again with this new value.
+  /// In other words, any pending change notifications must be discarded.
+  // TODO(jmesserly): I don't like a getter with side effects. Should we just
+  // rename the getter/setter pair to discardChanges/setValue like they are in
+  // JavaScript?
+  get value;
+
+  /// This can be implemented for two-way bindings. By default does nothing.
+  set value(newValue) {}
+
+  /// Deliver changes. Typically this will perform dirty-checking, if any is
+  /// needed.
+  void deliver() {}
+}