thenAnswerInOrder support

[ANDREYDEN]

Currently, thenReturnInOrder does not accept Futures or Streams (similarly to thenReturn).

I would like to be able to write the following code:

when(methodReturningStream).thenAnswerInOrder([
    Stream.fromIterable([1, 2, 3]),
    Stream.fromIterable([4, 5, 6]),
]);

I've looked into the code and found that thenReturnInOrder internally actually calls thenAnswer, which probably means that the call to _throwIfInvalid is not needed.

This is similar to #260, but for Futures and Streams.

Edit: found this workaround, but it is still not trivial to figure this out when first coming to the package. I suppose an alternative to thenAnswerInOrder could be storing some state on the Invocation object:

when(methodReturningStream).thenAnswer((invocation) => [
    Stream.fromIterable([1, 2, 3]),
    Stream.fromIterable([4, 5, 6]),
][invocation.numberOfTimesCalled]);

I'm quite new to this library, so please correct me if anything I'm saying is wrong.