Rewrite mixin application handling in Fasta.

pkg/front_end/lib/src/fasta/modifier.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.modifier;
 
 import 'errors.dart' show internalError;
 
 enum ModifierEnum {
   Abstract,
   Const,
   External,
   Final,
   Static,
 
   // Not a real modifier.
   Var,
 }
 
 const int abstractMask = 1;
 
 const int constMask = abstractMask << 1;
 
 const int externalMask = constMask << 1;
 
 const int finalMask = externalMask << 1;
 
 const int staticMask = finalMask << 1;
 
-/// Not a real modifier, and by setting it to null, it is automatically
-/// ignored by [Modifier.validate] below.
+const int namedMixinApplicationMask = staticMask << 1;
+
+/// Not a real modifier, and by setting it to zero, it is automatically ignored
+/// by [Modifier.validate] below.
 const int varMask = 0;
 
 const Modifier Abstract = const Modifier(ModifierEnum.Abstract, abstractMask);
 
 const Modifier Const = const Modifier(ModifierEnum.Const, constMask);
 
 const Modifier External = const Modifier(ModifierEnum.External, externalMask);
 
 const Modifier Final = const Modifier(ModifierEnum.Final, finalMask);
 
@@ skipping 24 matching lines @@
   static int validate(List<Modifier> modifiers, {bool isAbstract: false}) {
     // TODO(ahe): Implement modifier validation: ordering and uniqueness.
     int result = isAbstract ? abstractMask : 0;
     if (modifiers == null) return result;
     for (Modifier modifier in modifiers) {
       result |= modifier.mask;
     }
     return result;
   }
 }