Fixed strong mode errors and warnings reachable from lib/observe.dart

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],
+@MirrorsUsed(
+    metaTargets: const [Reflectable, ObservableProperty],
     override: 'smoke.mirrors')
 import 'dart:mirrors' show MirrorsUsed;
 import 'package:observe/observe.dart' show Reflectable, ObservableProperty;