Fix strong mode errors introduced by b62a37a36b2d01c3c27979a2cf5eada24afce035.

pkg/front_end/lib/src/fasta/dill/dill_class_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.dill_class_builder;
 
 import 'package:kernel/ast.dart' show
     Class,
     Constructor,
     Member,
     Procedure,
     ProcedureKind;
 
 import '../errors.dart' show
     internalError;
 
 import '../kernel/kernel_builder.dart' show
     Builder,
     KernelClassBuilder,
-    TypeBuilder;
+    KernelTypeBuilder;
 
 import '../modifier.dart' show
     abstractMask;
 
 import 'dill_member_builder.dart' show
     DillMemberBuilder;
 
 import 'dill_library_builder.dart' show
     DillLibraryBuilder;
 
@@ skipping 20 matching lines @@
       } else {
         existing.next = builder;
       }
     }
   }
 
   /// Returns true if this class is the result of applying a mixin to its
   /// superclass.
   bool get isMixinApplication => cls.isMixinApplication;
 
-  TypeBuilder get mixedInType => internalError("Not implemented.");
+  KernelTypeBuilder get mixedInType => internalError("Not implemented.");
 
   Builder findConstructorOrFactory(String name) => constructors[name];
 }
 
 int computeModifiers(Class cls) {
   return cls.isAbstract ? abstractMask : 0;
 }