Ensure that nulls aren't added to constant eval dependency graph.

Ensure that nulls aren't added to constant eval dependency graph.

In r45541, I accidentally left out a null check causing nulls to get
inserted into the constant evaluation dependency graph.  This caused a
log message to be printed when running the command-line analyzer, but
no test failures.

I've inserted the proper null check and added an assert statement so
that if the bug recurs it will be caught during testing.

BUG=dartbug.com/23407
R=brianwilkerson@google.com

Committed: https://code.google.com/p/dart/source/detail?r=45594

[Paul Berry, 2015-05-07 13:46:56]

paulberry@google.com changed reviewers:
+ brianwilkerson@google.com

[Brian Wilkerson, 2015-05-07 14:20:53]

LGTM

[Paul Berry, 2015-05-07 14:42:08]

Committed patchset #1 (id:1) manually as 45594 (presubmit successful).