completion function
Matches a Future that completes succesfully with a value that matches
matcher.
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.
To test that a Future completes with an exception, you can use throws and throwsA.
This returns an AsyncMatcher, so expect won't complete until the matched
future does.
Implementation
Matcher completion(matcher, [@deprecated String description]) =>
_Completes(wrapMatcher(matcher));