GwtAstBuilder better handling of JSNI refs to constants.

Turns out the target GWT field is not always fully baked, as it was in GenerateJavaAST.  Using JDT to get the compile-time constant is the right approach.

http://gwt-code-reviews.appspot.com/1449818/

Review by: jbrosenberg@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10337 8db76d5a-ed1c-0410-87a9-c151d255dfc7