core: Replaced unmodifiable due to GWT-RPC bug

Apparently GWT-RPC serialization doesn't handle unmodifiable
collections well, see
https://code.google.com/p/google-web-toolkit/issues/detail?id=3328.

Change-Id: Ia9f13df5f9d2b742b2752ef8a0180f19252b602a
Signed-off-by: Lior Vernia <lvernia@redhat.com>