Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/observe/src/metadata.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.metadata;
+
+/// Use `@observable` to make a field automatically observable, or to indicate
+/// that a property is observable. This only works on classes that extend or
+/// mix in `Observable`.
+const ObservableProperty observable = const ObservableProperty();
+
+/// An annotation that is used to make a property observable.
+/// Normally this is used via the [observable] constant, for example:
+///
+///     class Monster extends Observable {
+///       @observable int health;
+///     }
+///
+// TODO(sigmund): re-add this to the documentation when it's really true:
+//     If needed, you can subclass this to create another annotation that will
+//     also be treated as observable.
+// Note: observable properties imply reflectable.
+class ObservableProperty {
+  const ObservableProperty();
+}
+
+
+/// This can be used to retain any properties that you wish to access with
+/// Dart's mirror system. If you import `package:observe/mirrors_used.dart`, all
+/// classes or members annotated with `@reflectable` wil be preserved by dart2js
+/// during compilation.  This is necessary to make the member visible to
+/// `PathObserver`, or similar systems, once the code is deployed, if you are
+/// not doing a different kind of code-generation for your app. If you are using
+/// polymer, you most likely don't need to use this annotation anymore.
+const Reflectable reflectable = const Reflectable();
+
+/// An annotation that is used to make a type or member reflectable. This makes
+/// it available to `PathObserver` at runtime. For example:
+///
+///     @reflectable
+///     class Monster extends ChangeNotifier {
+///       int _health;
+///       int get health => _health;
+///       ...
+///     }
+///     ...
+///       // This will work even if the code has been tree-shaken/minified:
+///       final monster = new Monster();
+///       new PathObserver(monster, 'health').changes.listen(...);
+class Reflectable {
+  const Reflectable();
+}