Only return eventhandler tokens in stacks of 8, to avoid runtime calls.

runtime/bin/eventhandler_macos.h

 // 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.
 
 #ifndef BIN_EVENTHANDLER_MACOS_H_
 #define BIN_EVENTHANDLER_MACOS_H_
 
 #if !defined(BIN_EVENTHANDLER_H_)
 #error Do not include eventhandler_macos.h directly; use eventhandler.h instead.
 #endif
@@ skipping 18 matching lines @@
 };
 
 
 class SocketData {
  public:
   explicit SocketData(intptr_t fd)
       : fd_(fd),
         port_(0),
         mask_(0),
         tracked_by_kqueue_(false),
-        tokens_(8) {
+        tokens_(16) {
     ASSERT(fd_ != -1);
   }
 
   bool HasReadEvent();
   bool HasWriteEvent();
 
   void Close() {
     port_ = 0;
     mask_ = 0;
     VOID_TEMP_FAILURE_RETRY(close(fd_));
@@ skipping 65 matching lines @@
   TimeoutQueue timeout_queue_;
   bool shutdown_;
   int interrupt_fds_[2];
   int kqueue_fd_;
 };
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // BIN_EVENTHANDLER_MACOS_H_