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/NASHORN-668.js:34:15 Repeated RegExp flag: g
var x = /xxx/gg;
^
test/script/error/NASHORN-668.js:36:14 Unsupported RegExp flag: X
var y = /xxx/X;
^
test/script/error/NASHORN-668.js:38:13 Dangling meta character '*' near index 2
2**
^
var z = /2**/;
^