Future< T>.microtask constructor
    Creates a future containing the result of calling computation
asynchronously with scheduleMicrotask.
If executing computation throws,
the returned future is completed with the thrown error.
If calling computation returns a Future, completion of
the created future will wait until the returned future completes,
and will then complete with the same result.
If calling computation returns a non-future value,
the returned future is completed with that value.
Implementation
factory Future.microtask(FutureOr<T> computation()) {
  _Future<T> result = new _Future<T>();
  scheduleMicrotask(() {
    try {
      result._complete(computation());
    } catch (e, s) {
      _completeWithErrorCallback(result, e, s);
    }
  });
  return result;
}