After the last round of changes, I discovered that the new assertion in JField is actually getting 
tripped when a JsniFieldRef is constructed.  To get around this, I initialize the instance field to a 
null literal during a JsniFieldRef's construction.  I also mirrored these changed into JMethod and 
JsniMethodRef.

Review by: mmendez



git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@359 8db76d5a-ed1c-0410-87a9-c151d255dfc7