fix part of #25200, reject non-generic function subtype of generic function

pkg/analyzer/lib/src/dart/element/element.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.src.dart.element.element;
 
 import 'dart:collection';
 import 'dart:math' show min;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 4582 matching lines @@
    */
   DartType bound;
 
   /**
    * Initialize a newly created method element to have the given [name] and
    * [offset].
    */
   TypeParameterElementImpl(String name, int offset) : super(name, offset);
 
   /**
+   * Initialize a newly created synthetic type parameter element to have the
+   * given [name], and with [synthetic] set to true.
+   */
+  TypeParameterElementImpl.synthetic(String name) : super(name, -1) {
+    synthetic = true;
+  }
+
+  /**
    * Initialize a newly created type parameter element to have the given [name].
    */
   TypeParameterElementImpl.forNode(Identifier name) : super.forNode(name);
 
   @override
   ElementKind get kind => ElementKind.TYPE_PARAMETER;
 
   @override
   accept(ElementVisitor visitor) => visitor.visitTypeParameterElement(this);
 
@@ skipping 170 matching lines @@
 
   @override
   void visitElement(Element element) {
     int offset = element.nameOffset;
     if (offset != -1) {
       map[offset] = element;
     }
     super.visitElement(element);
   }
 }