invalidate method

void invalidate ()

Removes any cached value.

Implementation 

void invalidate() {
  // TODO: This does not return a future, but probably should.
  _cachedValueFuture = null;
  // TODO: This does not await, but probably should.
  _cachedStreamSplitter?.close();
  _cachedStreamSplitter = null;
  _stale?.cancel();
  _stale = null;
}