The test java/awt/Toolkit/RealSync/Test.java occasionally crashes on macOS. The crash occurs in [NSApplicationAWT sendEvent:] using seenDummyEventLock with value nil.

I believe the source of the problem is that LWCToolkit.nativeSyncQueue is not thread safe. Here is an example of a possible sequence of events that could cause this problem:

* thread1 postDummyEvent: creates condition lock #1 (condition NO), records timestamp #1, and posts event #1
* thread1 waitForDummyEvent: waits for lock #1 condition to be YES or timeout
* AppKit thread receives event #1 with timestamp #1, sets lock #1 condition to YES
* thread2 postDummyEvent: creates condition lock #2 (condition NO), records timestamp #2 and posts event #2 [note that the condition lock #1 is now inaccessible and cannot be released]
* thread2 waitForDummyEvent: waits for lock #2 condition to be YES or timeout
* thread1 waitForDummyEvent: acquires lock #1 because the condition is YES
* thread1 waitForDummyEvent: unlocks, releases lock #2 [using the global variable seenDummyEventLock] and sets seenDummyEventLock to nil
* AppKit thread receives event with timestamp #2 — sendEvent: crashes because the global variable seenDummyEventLock is nil