We currently define Thread::as_Java_thread() as an instance method to convert from a Thread* to a JavaThread*. Other Thread conversions have opted for a cast-style approach e.g.

static WorkerThread::cast(Thread* _this);
static CompilerThread::cast(Thread* _this);

we should look at doing JavaThread conversions in the same way for consistency.

This also helps remove Thread's knowledge of its subtypes from its API.