matchAsync method

dynamic matchAsync (dynamic item)

Returns null if this matches item, or a String description of the failure if it doesn't match.

This can return a Future or a synchronous value. If it returns a Future, neither expect nor the test will complete until that Future completes.

If this returns a String synchronously, expect will synchronously throw a TestFailure and matches will synchronusly return false.

Implementation 

/*FutureOr<String>*/ matchAsync(item) {
  if (item is! Function && item is! Future) {
    return "was not a Function or Future";
  }

  if (item is Future) {
    return item.then((value) => indent(prettyPrint(value), first: 'emitted '),
        onError: _check);
  }

  try {
    var value = item();
    if (value is Future) {
      return value.then(
          (value) => indent(prettyPrint(value),
              first: 'returned a Future that emitted '),
          onError: _check);
    }

    return indent(prettyPrint(value), first: 'returned ');
  } catch (error, trace) {
    return _check(error, trace);
  }
}