Implement type variables in mixin applications.

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 1ad94b21391efc7aa1f7b3b15e34ed8c5bb39c04..5fedd95364caf67e37b044761da29005ca5a58c1 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
@@ -16,6 +16,7 @@ import 'kernel_builder.dart' show
     LibraryBuilder,
     MetadataBuilder,
     NamedMixinApplicationBuilder,
+    TypeBuilder,
     TypeVariableBuilder;
 
 class KernelNamedMixinApplicationBuilder extends SourceClassBuilder
@@ -29,4 +30,8 @@ class KernelNamedMixinApplicationBuilder extends SourceClassBuilder
           interfaces, <String, Builder>{}, parent, null, charOffset);
 
   KernelTypeBuilder get mixinApplication => supertype;
+
+  // 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;
 }