Set -sourceLevel when precompiling modules

Without -sourceLevel, the gwtar files would depend on the JDK used to
compile GWT. We want GWT to be compatible with Java 6 JVMs so the gwtar
files must be built with -sourceLevel 1.6 whichever the JDK used to
produce the GWT artifacts.

Change-Id: I69250d554f66615aa2d99f44c612d5eba0d5a8b8