fixes bug in getAnnotations which should be side-effect free.

Then a small refactoring: the result of getAnnotation is now immutable (unmodifiable list).

git-svn-id: svn://scm.gforge.inria.fr/svnroot/spoon/trunk/spoon-core@2377 b1f50995-73fd-43b8-8ca2-fbc405a04e82