Fix strong mode errors introduced by b62a37a36b2d01c3c27979a2cf5eada24afce035.

pkg/front_end/lib/src/fasta/dill/dill_class_builder.dart

Index: pkg/front_end/lib/src/fasta/dill/dill_class_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/dill/dill_class_builder.dart b/pkg/front_end/lib/src/fasta/dill/dill_class_builder.dart
index 6511ca85838093e6c985c985ab21755ec216fc8b..2c4df33205fa5e0a93bb6b33ecccccd5fa50a7ae 100644
--- a/pkg/front_end/lib/src/fasta/dill/dill_class_builder.dart
+++ b/pkg/front_end/lib/src/fasta/dill/dill_class_builder.dart
@@ -17,7 +17,7 @@ import '../errors.dart' show
 import '../kernel/kernel_builder.dart' show
     Builder,
     KernelClassBuilder,
-    TypeBuilder;
+    KernelTypeBuilder;
 
 import '../modifier.dart' show
     abstractMask;
@@ -58,7 +58,7 @@ class DillClassBuilder extends KernelClassBuilder {
   /// superclass.
   bool get isMixinApplication => cls.isMixinApplication;
 
-  TypeBuilder get mixedInType => internalError("Not implemented.");
+  KernelTypeBuilder get mixedInType => internalError("Not implemented.");
 
   Builder findConstructorOrFactory(String name) => constructors[name];
 }