completes top-level property Null safety
Matches a Future that completes successfully with any value.
This creates an asynchronous expectation. The call to expect will return
immediately and execution will continue. Later, when the future completes,
the expectation against
matcher will run. To wait for the future to
complete and the expectation to run use expectLater and wait on the
final Matcher completes = const _Completes(null);