Always mark stdout as blocking when using 'print'.

runtime/bin/platform_win.cc

Index: runtime/bin/platform_win.cc
diff --git a/runtime/bin/platform_win.cc b/runtime/bin/platform_win.cc
index 85b6694d47dba42a51d60eb42c083047b95ec09f..cd0f0f285683033a63bf67d87ff6fb9be6cf597c 100644
--- a/runtime/bin/platform_win.cc
+++ b/runtime/bin/platform_win.cc
@@ -64,6 +64,16 @@ void Platform::FreeEnvironment(char** env, int count) {
   delete[] env;
 }
 
+
+void Platform::PrintBlocking(FILE* file, const char* format, ...) {
+  // No need to set blocking, as it's always blocking on Windows.
+  va_list args;
+  va_start(args, format);
+  vfprintf(file, format, args);
+  fflush(file);
+  va_end(args);
+}
+
 }  // namespace bin
 }  // namespace dart