Rewrite mixin application handling in Fasta.

pkg/front_end/lib/src/fasta/modifier.dart

Index: pkg/front_end/lib/src/fasta/modifier.dart
diff --git a/pkg/front_end/lib/src/fasta/modifier.dart b/pkg/front_end/lib/src/fasta/modifier.dart
index ff895e8039da6d6e48e040fdf0ee27123efae2e6..ac6fce6f393e7af7b4a926271e46ac10d3c0c3ee 100644
--- a/pkg/front_end/lib/src/fasta/modifier.dart
+++ b/pkg/front_end/lib/src/fasta/modifier.dart
@@ -27,8 +27,10 @@ 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);