Handle creating ParameterMirrors for the parameters of forwarding constructors.

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index c3e77eff330d9b87605ed479d00ea5f0b74d7378..897f2354581a7037b4c9fc7bb69de938d4c633a6 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -1577,6 +1577,11 @@ class Function : public Object {
   RawFunction* RedirectionTarget() const;
   void SetRedirectionTarget(const Function& target) const;
 
+  // Target information for forwarding constructors of mixin applications.
+  bool IsForwardingConstructor() const;
+  RawFunction* ForwardingTarget() const;
+  void SetForwardingTarget(const Function& target) const;
+
   RawFunction::Kind kind() const {
     return KindBits::decode(raw_ptr()->kind_tag_);
   }