ThreadLocal does not implement initialValue()

[ronshapiro]

ThreadLocal is missing the initialValue() method from the JDK. I'd like to add it.

Current GWT code: https://github.com/gwtproject/gwt/blob/4825d47eee7505346de372a159db55b18b006a2a/user/super/com/google/gwt/emul/java/lang/ThreadLocal.java

JDK API: https://docs.oracle.com/javase/7/docs/api/java/lang/ThreadLocal.html#initialValue()

[jnehlmeier]

https://gwt-review.googlesource.com/c/gwt/+/21160

[niloc132]

If we're still aiming to land this in 2.9, please follow up on the review feedback.

[niloc132]

Unmarking as "review pending", but the above link can be used to get the patch remade as a PR (and updated to be finished).