Check for a NULL value before passing it to a function that assumes non-NULL.

Check for a NULL value before passing it to a function that assumes non-NULL.


R=vegorov@google.com
BUG=

Committed: https://code.google.com/p/dart/source/detail?r=12776

[Vyacheslav Egorov (Google), 2012-09-24 12:55:39]

lgtm