Terminate event handler thread on isolate shutdown.

runtime/bin/eventhandler_macos.h

Index: runtime/bin/eventhandler_macos.h
diff --git a/runtime/bin/eventhandler_macos.h b/runtime/bin/eventhandler_macos.h
index 0c1c86d00697905efcf09e0d8b5cfce04fd2cc37..b2955331fabfd0a931c46898704fa41d67bb45af 100644
--- a/runtime/bin/eventhandler_macos.h
+++ b/runtime/bin/eventhandler_macos.h
@@ -107,6 +107,7 @@ class EventHandlerImplementation {
   SocketData* GetSocketData(intptr_t fd);
   void SendData(intptr_t id, Dart_Port dart_port, intptr_t data);
   void StartEventHandler();
+  void Shutdown();
 
  private:
   intptr_t GetTimeout();
@@ -124,6 +125,7 @@ class EventHandlerImplementation {
   HashMap socket_map_;
   int64_t timeout_;  // Time for next timeout.
   Dart_Port timeout_port_;
+  bool shutdown_;
   int interrupt_fds_[2];
   int kqueue_fd_;
 };