RFR: 8156619: Unimplemented ES6 features should result in clear Error being thrown

[Hannes Wallnöfer]

Please review:

Bug: https://bugs.openjdk.java.net/browse/JDK-8156619
Webrev: http://cr.openjdk.java.net/~hannesw/8156619/webrev.00/

Note that this also contains a small fix in Parser which was an oversight in my previous backport of ES6 changes from Graaljs. It made some destructuring cases impossible to detect but does not affect the syntax we currently support, which is why it wasn’t noticed before.

Thanks,
Hannes