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

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

Index: pkg/analyzer/lib/src/dart/element/element.dart
diff --git a/pkg/analyzer/lib/src/dart/element/element.dart b/pkg/analyzer/lib/src/dart/element/element.dart
index 08fdb84d60def544a1b6124fb22ff29c476aa936..07e45f86b767b8fc4bb4bb8a776462571c6e348b 100644
--- a/pkg/analyzer/lib/src/dart/element/element.dart
+++ b/pkg/analyzer/lib/src/dart/element/element.dart
@@ -4600,6 +4600,14 @@ class TypeParameterElementImpl extends ElementImpl
   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);