Modifies assertion for possibly null code pointer.

Modifies assertion for possibly null code pointer.

A callee function's code pointer may be nulled by a
GC pass during inlining. The code itself is still
around because the inlining function retains a
reference, but a future change will ensure that
the GC pass doesn't null code pointers in inlined
functions.

R=srdjan@google.com

Committed: https://code.google.com/p/dart/source/detail?r=29400

[srdjan, 2013-10-28 21:04:55]

lgtm

[zra, 2013-10-28 21:58:13]

Committed patchset #2 manually as r29400 (presubmit successful).