Better tests for the Parser / Preparser unification.

We need a way to assert that for a given source code snippet, an error *is*
produced or *is not* produced. Otherwise we might accidentally create new
errors or start accepting code which was previously not accepted. Just checking
that Parser and PreParser produce the same result doesn't cut it.

BUG=3126
LOG=N
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/154243005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@19107 ce2b1a6d-e550-0410-aec6-3dcde31c8c00