Always mark stdout and stderr as blocking on exit.

runtime/bin/platform_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/platform.h"
 
 #include <crt_externs.h>  // NOLINT
@@ skipping 64 matching lines @@
   int fd = fileno(file);
   FDUtils::SetBlocking(fd);
   va_list args;
   va_start(args, format);
   vfprintf(file, format, args);
   fflush(file);
   va_end(args);
   FDUtils::SetNonBlocking(fd);
 }
 
+
+void Platform::Cleanup() {
+  FDUtils::SetBlocking(fileno(stdout));
+  FDUtils::SetBlocking(fileno(stderr));
+}
+
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_MACOS)