Always mark stdout as blocking when using 'print'.

runtime/bin/platform_macos.cc

Index: runtime/bin/platform_macos.cc
diff --git a/runtime/bin/platform_macos.cc b/runtime/bin/platform_macos.cc
index 0576c345df54453ffc17ec408c1672ddfc6407ed..5af6c5ac643c2fbdfa17b6706fa80bffa319aa32 100644
--- a/runtime/bin/platform_macos.cc
+++ b/runtime/bin/platform_macos.cc
@@ -12,6 +12,8 @@
 #include <string.h>  // NOLINT
 #include <unistd.h>  // NOLINT
 
+#include "bin/fdutils.h"
+
 
 namespace dart {
 namespace bin {
@@ -68,6 +70,18 @@ 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