Simple bugfix in reaching definitions.
Simple bugfix in reaching definitions.

We only track definitions of stack-allocated variables.  Trying to
look up other variables in the environment is a bug.

Review URL: http://codereview.chromium.org/843006

git-svn-id: b158db1e4b4ab85d4c9e510fdef4b1e8c614b15b@4119 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh