--release=8 doesn't allow access to internal APIs

from http://mail.openjdk.java.net/pipermail/compiler-dev/2018-January/011583.html

> the spec for --release N is to provide access to the
> public documented API for JDK N.
>
> That being said, there has been discussion recently about providing
> access to some of the API that precedes jdk.unsupported, as a special
> case.

The current behaviour prevents migrating some uses of -source/-target to --release. Providing optional access to unsupported APIs when using --release=8 (perhaps in the spirit of -XDignore.symbol.file=true) would allow those uses of -source/-target to be migrated and to benefit from the better ergonomics of --release.