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

lib/mirrors_used.dart

Index: lib/mirrors_used.dart
diff --git a/lib/mirrors_used.dart b/lib/mirrors_used.dart
index b3d3f433913dc3270bf331392cd2b58caccce4ad..53878a19db48ee393f49e547415fac6c4e3221d7 100644
--- a/lib/mirrors_used.dart
+++ b/lib/mirrors_used.dart
@@ -12,7 +12,8 @@ library observe.mirrors_used;
 // 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;