RFR: 8224974: Implement JEP 352

[Brian Burkhalter]

To clarify, the following comments were for the _internal_ class. These along with a number of my other comments are just stylistic and not absolutely necessary to change.

Thanks,

Brian

> On Jun 5, 2019, at 7:36 AM, Brian Burkhalter <brian.burkhalter at oracle.com> wrote:
> 
> ExtendedMapMode
> 13:	Constant name should be MAP_MODE_CONSTRUCTOR.
> 13:	Insert blank line after
> 34-36:	Move to before line 13.
> 24:	Move constructor to end of class.