Prevent NPE in TypeAnalyzer

Prevent NPE in TypeAnalyzer

This came from a bug report in the editor without a 
specific repro case, so I just added protection
at the several places on the line where fetching the 
type arguments might hit a null pointer.  If I had
a repro case, I would work on preventing the null pointer
from getting in there in the first place.

http://code.google.com/p/dart/issues/detail?id=2365

Committed: https://code.google.com/p/dart/source/detail?r=6140

[scheglov, 2012-04-03 19:00:40]

LGTM