Add default type arguments on Supertype as well.

pkg/front_end/lib/src/fasta/kernel/kernel_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.kernel_builder;
 
 export 'kernel_class_builder.dart' show KernelClassBuilder;
 
 export 'kernel_enum_builder.dart' show KernelEnumBuilder;
 
@@ skipping 40 matching lines @@
         DartType,
         DynamicType,
         Field,
         Library,
         Member,
         Procedure,
         TypeParameter;
 
 import '../errors.dart' show inputError;
 
-List<DartType> computeDefaultTypeArguments(
+import '../builder/builder.dart' show LibraryBuilder;
+
+List<DartType> computeDefaultTypeArguments(LibraryBuilder library,
     List<TypeParameter> typeParameters, List<DartType> arguments) {
+  // TODO(ahe): Not sure what to do if `arguments.length !=
+  // cls.typeParameters.length`.
   if (arguments == null) {
     return new List<DartType>.filled(
         typeParameters.length, const DynamicType());
   }
   if (arguments.length < typeParameters.length) {
     arguments = new List<DartType>.from(arguments);
     for (int i = arguments.length; i < typeParameters.length; i++) {
       arguments.add(const DynamicType());
     }
   } else if (arguments.length > typeParameters.length) {
@@ skipping 22 matching lines @@
   charOffset ??= -1;
   if (charOffset == -1) {
     charOffset = member.fileOffset ?? -1;
   }
   if (charOffset == -1) {
     charOffset = cls?.fileOffset ?? -1;
   }
   name = (cls == null ? "" : "${cls.name}::") + name;
   return inputError(uri, charOffset, "Error in $name: $error");
 }