Always mark stdout as blocking when using 'print'.

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
 #include <signal.h>  // NOLINT
 #include <string.h>  // NOLINT
 #include <unistd.h>  // NOLINT
 
+#include "bin/fdutils.h"
+
 
 namespace dart {
 namespace bin {
 
 bool Platform::Initialize() {
   // Turn off the signal handler for SIGPIPE as it causes the process
   // to terminate on writing to a closed pipe. Without the signal
   // handler error EPIPE is set instead.
   struct sigaction act;
   bzero(&act, sizeof(act));
@@ skipping 36 matching lines @@
     result[current] = environ[current];
   }
   return result;
 }
 
 
 void Platform::FreeEnvironment(char** env, intptr_t count) {
   delete[] env;
 }
 
+
+void Platform::PrintBlocking(FILE* file, const char* format, ...) {
+  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);
+}
+
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_MACOS)