Add default type arguments on Supertype as well.

pkg/front_end/lib/src/fasta/kernel/kernel_builder.dart

Index: pkg/front_end/lib/src/fasta/kernel/kernel_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_builder.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_builder.dart
index f828c7e8f0acd79cfe80499c99ea8db587635c46..d7486c99a62baddc322c194bf65f3be9abcc176f 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_builder.dart
@@ -58,8 +58,12 @@ import 'package:kernel/ast.dart'
 
 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());