Fix use of generic function type aliases for fields (issue 29237)

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 5215 matching lines @@
   ElementImpl getChild(String identifier) {
     for (TypeParameterElement typeParameter in typeParameters) {
       TypeParameterElementImpl typeParameterImpl = typeParameter;
       if (typeParameterImpl.identifier == identifier) {
         return typeParameterImpl;
       }
     }
     return null;
   }
 
+  /**
+   * Return the type of the function defined by this typedef after substituting
+   * the given [typeArguments] for the type parameters defined for this typedef
+   * (but not the type parameters defined by the function). If the number of
+   * [typeArguments] does not match the number of type parameters, then
+   * `dynamic` will be used in place of each of the type arguments.
+   */
+  FunctionType typeAfterSubstitution(List<DartType> typeArguments) {
+    FunctionType functionType = function.type;
+    List<TypeParameterElement> parameterElements = typeParameters;
+    List<DartType> parameterTypes =
+        TypeParameterTypeImpl.getTypes(parameterElements);
+    int parameterCount = parameterTypes.length;
+    if (typeArguments == null ||
+        parameterElements.length != typeArguments.length) {
+      DartType dynamicType = DynamicElementImpl.instance.type;
+      typeArguments = new List<DartType>.filled(parameterCount, dynamicType);
+    }
+    return functionType.substitute2(typeArguments, parameterTypes);
+  }
+
   @override
   void visitChildren(ElementVisitor visitor) {
     super.visitChildren(visitor);
     safelyVisitChildren(typeParameters, visitor);
     function?.accept(visitor);
   }
 }
 
 /**
  * A concrete implementation of a [HideElementCombinator].
@@ skipping 3838 matching lines @@
 
   @override
   void visitElement(Element element) {
     int offset = element.nameOffset;
     if (offset != -1) {
       map[offset] = element;
     }
     super.visitElement(element);
   }
 }