Handle mandatory arguments correctly in function type aliases.

pkg/front_end/lib/src/fasta/builder/formal_parameter_builder.dart

 // Copyright (c) 2016, 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 fasta.formal_parameter_builder;
 
 import 'package:front_end/src/fasta/parser/parser.dart' show
     FormalParameterType;
 
 import 'builder.dart' show
@@ skipping 16 matching lines @@
   final bool hasThis;
 
   FormalParameterType kind = FormalParameterType.REQUIRED;
 
   FormalParameterBuilder(this.metadata, this.modifiers, this.type, this.name,
       this.hasThis, LibraryBuilder compilationUnit, int charOffset)
       : super(compilationUnit, charOffset);
 
   bool get isRequired => kind.isRequired;
 
-  bool get isPositional => kind.isPositional;
+  bool get isPositional => kind.isPositional || kind.isRequired;
 
   bool get isNamed => kind.isNamed;
 
   bool get isOptional => !isRequired;
 
   bool get isLocal => true;
 }