delete observe from the repo

pkg/observe/lib/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;