Make sure variable names used in status files are available in the environment

Make sure variable names used in status files are available in the environment

Previously we didn't validate that a variable name used in a status file
expression was available.

It wasn't surfaced, since lookups in Map objects return null if the key is
missing and calling 'toString()' on 'null' doesn't throw an exception.

BUG=http://dartbug.com/12959
R=ricow@google.com

Committed: https://code.google.com/p/dart/source/detail?r=30502

[ricow1, 2013-11-21 09:52:54]

LGTM

[kustermann, 2013-11-21 09:54:43]

Committed patchset #1 manually as r30502 (presubmit successful).