[typesystem] Allow to access some services that have to be injected redundantly otherwise

Change-Id: I2232f5f343abf09ef60a5e9953233c637f8ba986
[xtext][builder] Don’t try to build for project that did not have the Xtext nature

see https://bugs.eclipse.org/bugs/show_bug.cgi?id=455477

Change-Id: I8564f28a8e6839682970531d7f185b9aa7ecbf26