Tighten check disallowing use of initializing formals (fixes #26855).

runtime/vm/parser.h

Index: runtime/vm/parser.h
diff --git a/runtime/vm/parser.h b/runtime/vm/parser.h
index e852456c62f5cac79e1638f80acef414213dd69b..7798edb2cbcbe2c094d71e7298a7f91368affe17 100644
--- a/runtime/vm/parser.h
+++ b/runtime/vm/parser.h
@@ -671,8 +671,7 @@ class Parser : public ValueObject {
   ClosureNode* CreateImplicitClosureNode(const Function& func,
                                          TokenPosition token_pos,
                                          AstNode* receiver);
-  static void AddFormalParamsToFunction(const ParamList* params,
-                                        const Function& func);
+  void AddFormalParamsToFunction(const ParamList* params, const Function& func);
   void AddFormalParamsToScope(const ParamList* params, LocalScope* scope);
 
   SequenceNode* ParseConstructor(const Function& func);