Shrink AOT snapshot size and memory usage.

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{
-  Exception get exception;
+  get exception;
   /// [optional]
   StackTrace get stacktrace;
 }
 
 abstract class EventNotification implements Notification{
   Event get event;
 }