Review request for JDK-8009230

[Hannes Wallnoefer]

Please review webrev for JDK-8009230: Nashorn rejects extended RegExp 
syntax accepted by all major JS engines.

http://cr.openjdk.java.net/~hannesw/8009230/

We now produce the same result as Spidermonkey for the test script. V8 
behaves different if an invalid \x or \u escape is encountered at the 
end of the regexp, but I didn't really understand the motivation behind 
this and didn't follow it.

Hannes