Avoid an assert fault in snapshot writer by making sure that the parent of a

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index 5c007ee2624704b82e13061fbe466fb8889d2c1c..f08fa72fcf454c50c89afe7f61adad683f9680e1 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -2218,6 +2218,9 @@ class Function : public Object {
   RawType* SignatureType() const;
   RawType* ExistingSignatureType() const;
 
+  // Allocate and return a signature function equivalent to this function.
+  RawFunction* CanonicalSignatureFunction(TrailPtr trail) const;
+
   // Update the signature type (with a canonical version).
   void SetSignatureType(const Type& value) const;