Add attributions so printf like functions can have their arguments checked.

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index b2b665afa71518cab5551c9f0a5c0166728db680..cd733585a351685055242f29ec87ddb0c5ffef1a 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -3540,7 +3540,8 @@ class String : public Instance {
   static RawString* ToLowerCase(const String& str,
                                 Heap::Space space = Heap::kNew);
 
-  static RawString* NewFormatted(const char* format, ...);
+  static RawString* NewFormatted(const char* format, ...)
+      PRINTF_ATTRIBUTE(1, 2);
 
  protected:
   bool HasHash() const {