Set and keep parent function of signature functions.

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index af7a25b15721d68373692dbf668c195847a9eb94..a54e2f7eab87d8669cb930aacd1af1a70b970167 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -2856,7 +2856,9 @@ class Function : public Object {
 
   // Allocates a new Function object representing a signature function.
   // The owner is the scope class of the function type.
+  // The parent is the enclosing function or null if none.
   static RawFunction* NewSignatureFunction(const Object& owner,
+                                           const Function& parent,
                                            TokenPosition token_pos,
                                            Heap::Space space = Heap::kOld);