How can I mock the indexing operator with dart mockito?

I am writing a unit test in which I need to mock a JsObject so that I don't need to do actual javascript interop in my test. However, I'm using the indexing operator [] to access a field in my JsObject. I am using the dart mockito library http://ift.tt/1JgV2VI for mocking and I can't seem to find how I go about mocking the behavior of operators on the mocked object.