Fixed observatory_ui/vm_connect test in checked mode

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 43 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;
+    return const TargetMock();
   }
 
   @override
   bool isConnectedVMTarget(M.Target target) {
     return false;
   }
 
   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;
 }