Call known TLS destructors on Windows

runtime/vm/thread_test.cc

Index: runtime/vm/thread_test.cc
diff --git a/runtime/vm/thread_test.cc b/runtime/vm/thread_test.cc
index 8e78dcadc120f7cb316d168ef68ddfbb9cfbc0af..6f6075dea1ce85e2a510b45fbf6be482c30d9bbc 100644
--- a/runtime/vm/thread_test.cc
+++ b/runtime/vm/thread_test.cc
@@ -479,6 +479,8 @@ void ThreadIteratorTestMain(uword parameter) {
 }
 
 
+// NOTE: This test case also verifies that known TLS destructors are called
+// on Windows. See |OnThreadExit| in os_thread_win.cc for more details.
 TEST_CASE(ThreadIterator_AddFindRemove) {
   Isolate* isolate = thread->isolate();
   ThreadIteratorTestParams params;