[mini_types] Decouple NamedFunctionParameter from NamedType.

Previously, NamedFunctionParameter was derived from NamedType. This
avoided a tiny amount of code duplication, but the benefit was really
tiny. Decoupling the two classes simplifies the implementation of a
follow-up CL that I have in progress, which will allow the mini_types
framework to represent function types.

Change-Id: I29bff36ab76bbf344cf7204f2503b006a88a4cf8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/395422
Reviewed-by: Kallen Tu <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

pkg/_fe_analyzer_shared/test/mini_types.dart

@@ -180,13 +180,19 @@ class InvalidType extends _SpecialSimpleType
 }
 
 /// A named parameter of a function type.
-class NamedFunctionParameter extends NamedType
+class NamedFunctionParameter
     implements SharedNamedFunctionParameterStructure<Type> {
+  @override
+  final String name;
+
+  @override
+  final Type type;
+
   @override
   final bool isRequired;
 
   NamedFunctionParameter(
-      {required this.isRequired, required super.name, required super.type});
+      {required this.isRequired, required this.name, required this.type});
 
   @override
   String toString() => [if (isRequired) 'required', type, name].join(' ');