Yield as contextual keyword

[Doug Lea]

I don't enjoy being the token curmudgeon here, but I find it
increasingly hard to appreciate why a non-ambiguous choice (prefix "^")
with precedence in related languages should be rejected in favor of one
requiring context-sensitive grammar mangling with some known odd
consequences. At the very least, could someone help check as-yet-unknown
impact by using candidate parsers on large source corpuses (for example
http://groups.inf.ed.ac.uk/cup/javaGithub/, google-internal, etc)?

-Doug