Correctly revert eventhandler_macos.

runtime/bin/eventhandler_macos.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "platform/globals.h"
 #if defined(TARGET_OS_MACOS)
 
 #include "bin/eventhandler.h"
 
 #include <errno.h>  // NOLINT
@@ skipping 321 matching lines @@
     if ((events[i].flags & EV_ERROR) != 0) {
       const int kBufferSize = 1024;
       char error_message[kBufferSize];
       strerror_r(events[i].data, error_message, kBufferSize);
       FATAL1("kevent failed %s\n", error_message);
     }
     if (events[i].udata == NULL) {
       interrupt_seen = true;
     } else {
       SocketData* sd = reinterpret_cast<SocketData*>(events[i].udata);
-      RemoveFromKqueue(kqueue_fd_, sd);
       intptr_t event_mask = GetEvents(events + i, sd);
       if (event_mask != 0) {
+        // Unregister events for the file descriptor. Events will be
+        // registered again when the current event has been handled in
+        // Dart code.
+        RemoveFromKqueue(kqueue_fd_, sd);
         Dart_Port port = sd->port();
         ASSERT(port != 0);
         DartUtils::PostInt32(port, event_mask);
       }
     }
   }
   if (interrupt_seen) {
     // Handle after socket events, so we avoid closing a socket before we handle
     // the current events.
     HandleInterruptFd();
@@ skipping 92 matching lines @@
 
 uint32_t EventHandlerImplementation::GetHashmapHashFromFd(intptr_t fd) {
   // The hashmap does not support keys with value 0.
   return dart::Utils::WordHash(fd + 1);
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_MACOS)