Changed implementation of pushback in preparer character stream.
Removed assert that isn't satisfied by in test-cases, but only by the real code.
Make preparser compile again.

Review URL: http://codereview.chromium.org/5593004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5946 ce2b1a6d-e550-0410-aec6-3dcde31c8c00