Use dartfmt on Observatory code

runtime/observatory/lib/src/models/objects/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 models;
 
 abstract class Notification {}
 
-abstract class ExceptionNotification implements Notification{
+abstract class ExceptionNotification implements Notification {
   get exception;
+
   /// [optional]
   StackTrace get stacktrace;
 }
 
-abstract class EventNotification implements Notification{
+abstract class EventNotification implements Notification {
   Event get event;
 }