Fix strong mode errors introduced by b62a37a36b2d01c3c27979a2cf5eada24afce035.

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

Index: pkg/front_end/lib/src/fasta/kernel/kernel_named_mixin_application_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_named_mixin_application_builder.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_named_mixin_application_builder.dart
index 5fedd95364caf67e37b044761da29005ca5a58c1..38766f1546d6a2983ee8465190db7eaf0dd4d071 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_named_mixin_application_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_named_mixin_application_builder.dart
@@ -16,7 +16,6 @@ import 'kernel_builder.dart' show
     LibraryBuilder,
     MetadataBuilder,
     NamedMixinApplicationBuilder,
-    TypeBuilder,
     TypeVariableBuilder;
 
 class KernelNamedMixinApplicationBuilder extends SourceClassBuilder
@@ -33,5 +32,5 @@ class KernelNamedMixinApplicationBuilder extends SourceClassBuilder
 
   // TODO(ahe): This is a bit odd, as it means this answers false to
   // [isMixinApplication], but its superclass is the mixin application.
-  TypeBuilder get mixedInType => null;
+  KernelTypeBuilder get mixedInType => null;
 }