Use dartfmt on Observatory code

runtime/observatory/lib/src/app/notification.dart

 // Copyright (c) 2016, 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.
 
 part of app;
 
 class ExceptionNotification implements M.ExceptionNotification {
   final exception;
+
   /// [optional]
   final StackTrace stacktrace;
   ExceptionNotification(this.exception, {this.stacktrace});
 }
 
 class EventNotification implements M.EventNotification {
   final M.Event event;
   EventNotification(this.event);
 }