fix GWT compatibility of xbase/xtend libs

Change-Id: Ic4d2ed99164ab4e4a8fc1c3a86b8f62fccccee36
[xbase] make JvmModelAssociator thread safe
https://bugs.eclipse.org/bugs/show_bug.cgi?id=450148

Change-Id: I35f19f0579192687d3116ae0636fe231da0b8f6a
Merge "fix GWT compatibility of xbase/xtend libs"