Incubator Async Button not to be found?

[mcaap]

Hi,

Thanks for the wonderful GWT Material Design library. I could not find the Async widget in code, thought it is present in the demo site and demo code: https://gwtmaterialdesign.github.io/gwt-material-demo/?locale=fr#async

https://github.com/GwtMaterialDesign/gwt-material-demo/blob/ef9ede80abac6e5fed4cbe0ede772eb44f674c71/src/main/java/gwt/material/design/demo/client/application/incubator/async/AsyncView.java

Specifically I am looking for AsyncButton.

Thanks