Rewrite mixin application handling in Fasta.

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, Member;
 
 import '../errors.dart' show internalError;
 
@@ skipping 35 matching lines @@
     } else {
       scopeBuilder.addMember(name, builder);
     }
   }
 
   /// Returns true if this class is the result of applying a mixin to its
   /// superclass.
   bool get isMixinApplication => cls.isMixinApplication;
 
   KernelTypeBuilder get mixedInType => internalError("Not implemented.");
+
+  void set mixedInType(KernelTypeBuilder mixin) {
+    internalError("Not implemented.");
+  }
 }
 
 int computeModifiers(Class cls) {
   return cls.isAbstract ? abstractMask : 0;
 }