Scanner: disallow unicode escapes in regexp flags.

The spec explicitly forbids them. V8 never handled them properly either, just
the Scanner accepted them (it had code to add them literally to the
LiteralBuffer) and later on, Regexp constructor disallowed them.

According to the spec, unicode escapes in regexp flags should be an early error
("It is a Syntax Error if IdentifierPart contains a Unicode escape sequence.").

Note that Scanner is still more relaxed about regexp flags than the
spec. Especially, it accepts any identifier parts (not just a small set of
letters) and doesn't check for duplicates.

R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/700373003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25215 ce2b1a6d-e550-0410-aec6-3dcde31c8c00