This does not make a sense. Your `createLoopBackConnection` method always returns null which makes it not really good test mock.

Instead, you should have:

```dart

//given

var loopBackConnection = new Mock();
var createLoopBackConnection = new Mock()
  ..when(callsTo('call')).alwaysReturn(loopBackConnection);
var requestHandler = new MultiRequestHandler(createLoopBackConnection);

```

etc, etc, etc.