Annotations and java seq

That's not proposed for merging yet, as it triggered other bugs.

@odersky can you please have a look on commits starting from b15bd30? 
Changes to typer also made typer not able to retype itself. It fails with a datarace: https://gist.github.com/DarkDimius/d2b5b5cb34e7c400c741 This seems to be unrelated with changes that were made, just triggered by additional code.

I'm not investigating the CyclicReferenceError's that are triggered during Erasure.