Matches a Future that completes succesfully with a value that matches
Note that this creates an asynchronous expectation. The call to
expect() that includes this will return immediately and execution will
continue. Later, when the future completes, the actual expectation will run.
This returns an
AsyncMatcher, so expect won't complete until the matched
Matcher completion(matcher, [@deprecated String description]) => _Completes(wrapMatcher(matcher));