Rewrite mixin application handling in Fasta.

pkg/front_end/lib/src/fasta/builder/named_mixin_application_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.named_mixin_application_builder;
-
-import 'builder.dart'
-    show
-        ClassBuilder,
-        LibraryBuilder,
-        MemberBuilder,
-        MetadataBuilder,
-        Scope,
-        TypeBuilder,
-        TypeVariableBuilder;
-
-abstract class NamedMixinApplicationBuilder<T extends TypeBuilder, R>
-    extends ClassBuilder<T, R> {
-  NamedMixinApplicationBuilder(
-      List<MetadataBuilder> metadata,
-      String name,
-      List<TypeVariableBuilder> typeVariables,
-      int modifiers,
-      T supertype,
-      List<T> interfaces,
-      LibraryBuilder parent,
-      int charOffset)
-      : super(
-            metadata,
-            modifiers,
-            name,
-            typeVariables,
-            supertype,
-            interfaces,
-            new Scope(<String, MemberBuilder>{}, <String, MemberBuilder>{},
-                parent.scope.withTypeVariables(typeVariables),
-                isModifiable: false),
-            new Scope(<String, MemberBuilder>{}, null, null,
-                isModifiable: false),
-            parent,
-            charOffset);
-
-  T get mixinApplication => supertype;
-}