Move part of scanner.* into scanner-base.* for reuse in preparser scanner.

Move part of scanner.* into scanner-base.* for reuse in preparser scanner.
Make checks.h not depend on flags.h or global.h (or anything else except
include/v8stdint.h). Only checks.cc has the dependencies (so another
implementation of checks.cc can be provided by the preparser).
Now files depending on checks.h (using ASSERT macros) can include it
directly without depending on all of v8.

Review URL: http://codereview.chromium.org/4576001

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5775 ce2b1a6d-e550-0410-aec6-3dcde31c8c00