Review request for JDK-8151700: Add support for ES6 for-of

[Hannes Wallnoefer]

Please review JDK-8151700: Add support for ES6 for-of:
http://cr.openjdk.java.net/~hannesw/8151700/webrev/

This enables ES6 for-of loops. The ES6 iterator is wrapped in a 
java.util.Iterator so it uses the exact same codegen infrastructure as 
for-in.

Hannes