[vm] Check for mis-matched thread type in OSThread TLS destructor.

Change-Id: I1d878d4d31877749cf47ddd3ef973386592fd714
Reviewed-on: https://dart-review.googlesource.com/42104
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Zach Anderson <zra@google.com>

runtime/vm/os_thread.cc

@@ -62,6 +62,12 @@ OSThread* OSThread::CreateOSThread() {
 }
 
 OSThread::~OSThread() {
+  if (!is_os_thread()) {
+    // If the embedder enters an isolate on this thread and does not exit the
+    // isolate, the thread local at thread_key_, which we are destructing here,
+    // will contain a dart::Thread instead of a dart::OSThread.
+    FATAL("Thread exited without calling Dart_ExitIsolate");
+  }
   RemoveThreadFromList(this);
   delete log_;
   log_ = NULL;