Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/observe/mirrors_used.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.
+
+/// An empty library that declares what needs to be retained with [MirrorsUsed]
+/// if you were to use observables together with mirrors. By default this is not
+/// included because frameworks using this package also use code generation to
+/// avoid using mirrors at deploy time.
+library observe.mirrors_used;
+
+// Note: ObservableProperty is in this list only for the unusual use case of
+// invoking dart2js without running this package's transformers. The
+// transformer in `lib/transformer.dart` will replace @observable with the
+// @reflectable annotation.
+@MirrorsUsed(metaTargets: const [Reflectable, ObservableProperty],
+    override: 'smoke.mirrors')
+import 'dart:mirrors' show MirrorsUsed;
+import 'package:observe/observe.dart' show Reflectable, ObservableProperty;