Don't use kFSEventStreamCreateFlagNoDefer as it messes with event flushing.

runtime/bin/file_system_watcher_macos.cc

 // Copyright (c) 2013, 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/file_system_watcher.h"
 
 #include <errno.h>  // NOLINT
@@ skipping 95 matching lines @@
       context.info = reinterpret_cast<void*>(node);
       CFArrayRef array = CFArrayCreate(
           NULL, reinterpret_cast<const void**>(&node->path_ref_), 1, NULL);
       FSEventStreamRef ref = FSEventStreamCreate(
           NULL,
           Callback,
           &context,
           array,
           kFSEventStreamEventIdSinceNow,
           0.10,
-          kFSEventStreamCreateFlagNoDefer | kFSEventStreamCreateFlagFileEvents);
+          kFSEventStreamCreateFlagFileEvents);
       CFRelease(array);
 
       node->set_ref(ref);
 
       FSEventStreamScheduleWithRunLoop(
           node->ref_,
           node->watcher_->run_loop_,
           kCFRunLoopDefaultMode);
 
       FSEventStreamStart(node->ref_);
@@ skipping 261 matching lines @@
   }
   return events;
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_MACOS)