Add product mode:

runtime/vm/flags.h

Index: runtime/vm/flags.h
diff --git a/runtime/vm/flags.h b/runtime/vm/flags.h
index f703c1000512f53324936c5531cc9c3326bd6c9b..d0b8fbaf04b97ce92b568492e185367e42b3a60d 100644
--- a/runtime/vm/flags.h
+++ b/runtime/vm/flags.h
@@ -19,6 +19,18 @@ typedef const char* charp;
                                             default_value,                     \
                                             comment)
 
+#if defined(PRODUCT)
+#define PRODUCT_FLAG(product_value, type, name, default_value, comment)        \
+  const type FLAG_##name = product_value;
+
+#else  // defined(PRODUCT)
+#define PRODUCT_FLAG(product_value, type, name, default_value, comment)        \
+  type FLAG_##name = Flags::Register_##type(&FLAG_##name,                      \
+                                            #name,                             \
+                                            default_value,                     \
+                                            comment)
+#endif  // defined(PRODUCT)
+
 #define DEFINE_FLAG_HANDLER(handler, name, comment)                            \
   bool DUMMY_##name = Flags::Register_func(handler, #name, comment)