Track async causal stack traces

runtime/observatory/tests/observatory_ui/mocks/repositories/target.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 mocks;
 
 class TargetChangeEventMock implements M.TargetChangeEvent {
   final TargetRepositoryMock repository;
   const TargetChangeEventMock({this.repository});
 }
@@ skipping 42 matching lines @@
 
   void delete(M.Target target) {
     deleteInvoked = true;
     if (_delete != null) _delete(target);
   }
 
   void triggerChangeEvent() {
     _onChange.add(new TargetChangeEventMock(repository: this));
   }
 
+  M.Target find(String networkAddress) {
+    return null;
+  }
+
   TargetRepositoryMock({M.Target current, Iterable<M.Target> list : const [],
       TargetRepositoryMockStringCallback add,
       TargetRepositoryMockTargetCallback setCurrent,
       TargetRepositoryMockTargetCallback delete})
     : _current = current,
       _list = list,
       _add = add,
       _setCurrent = setCurrent,
       _delete = delete;
 }