Tests and fixes for (pre)parse errors related to future reserved words.

This contains the following fixes:
- PreParser was using an error "reserved_word" which doesn't exist in
messages.js. Changed it to "unexpected_reserved".

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

Review URL: https://codereview.chromium.org/153793002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19076 ce2b1a6d-e550-0410-aec6-3dcde31c8c00