Issue 23190. Type check with a function type alias is its usage.

pkg/analyzer/lib/src/generated/resolver.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 engine.resolver;
 
 import "dart:math" as math;
 import 'dart:collection';
 
 import 'package:analyzer/src/generated/utilities_collection.dart';
@@ skipping 4548 matching lines @@
     }
     // ignore references to an element from itself
     if (identical(element, _enclosingClass)) {
       return;
     }
     if (identical(element, _enclosingExec)) {
       return;
     }
     // ignore places where the element is not actually used
     if (node.parent is TypeName) {
-      AstNode parent2 = node.parent.parent;
-      if (parent2 is IsExpression) {
-        return;
-      }
-      // We need to instantiate/extend/implement a class to actually use it.
-      // OTOH, function type aliases are used to define closure structures.
-      if (parent2 is VariableDeclarationList && element is ClassElement) {
-        return;
+      if (element is ClassElement) {
+        AstNode parent2 = node.parent.parent;
+        if (parent2 is IsExpression) {
+          return;
+        }
+        if (parent2 is VariableDeclarationList) {
+          return;
+        }
       }
     }
     // OK
     usedElements.addElement(element);
   }
 
   static bool _isReadIdentifier(SimpleIdentifier node) {
     // not reading at all
     if (!node.inGetterContext()) {
       return false;
@@ skipping 10822 matching lines @@
     nonFields.add(node);
     return null;
   }
 
   @override
   Object visitNode(AstNode node) => node.accept(TypeResolverVisitor_this);
 
   @override
   Object visitWithClause(WithClause node) => null;
 }