Disallow escaped characters in a regular expression flag.

https://code.google.com/p/esprima/issues/detail?id=503
Minor fix-up on the test for regular expression flag of escapes.

https://code.google.com/p/esprima/issues/detail?id=503
Update ChangeLog for 1.2.

https://code.google.com/p/esprima/issues/detail?id=514