I just cloned the example repository from the blog (https://github.com/csadilek/errai-mvp-demo), updated the version to 2.1 and it built fine. The two classes that cause problems for you should have been overwritten by our errai-javax-enterprise project which takes away those methods.
The maven dependency for this project (overriding the EE6 APIs) is:
This jar has to be on your classpath during compilation. For some reason when you build your project the original EE6 APIs are found first. Are there any modification you did to the pom?
Ah, that probably explains it. The code was copied from the blog post and sample app, but the POM was a case of "copy and paste the differences until it worked". I'll see if your suggestion fixes the issue on Monday.
For anyone else with the same error, this can be fixed by adding
to the App.gwt.xml file.