8023454: Updated DEVELOPER_README and command line flags, ensuring that undocumented flags that aren't guaranteed to work (disabled by default) and that are work in progress show up with an EXPERIMENTAL tag. Reviewed-by: attila, jlaskey

test/script/error/JDK-8020437-2.js:36:12 Expected ; but found ++
EOF}); var x++; // syntax error in same line
            ^