1. override
Iterable<OverlayEntry> createOverlayEntries()

Subclasses should override this getter to return the builders for the overlay.

Source 

@override
Iterable<OverlayEntry> createOverlayEntries() sync* {
  yield new OverlayEntry(builder: _buildModalBarrier);
  yield new OverlayEntry(builder: _buildModalScope, maintainState: maintainState);
}