RFR: JDK-8283415: Update java.lang.ref to use sealed classes

[Kim Barrett]

On Sat, 19 Mar 2022 22:30:13 GMT, Joe Darcy <darcy at openjdk.org> wrote:

> Another refactoring to use sealed classes where possible, this time in the java.lang.ref package.
> 
> Copyright dates will be updated, if needed, before a push.

Looks good.

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Marked as reviewed by kbarrett (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/7874