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 6fb751488bce61767c64cd494b4596345e6b0d69..4b2f5c6e18898939581b59165c63dd4138b38931 100644
--- a/pkg/front_end/lib/src/fasta/modifier.dart
+++ b/pkg/front_end/lib/src/fasta/modifier.dart
@@ -30,6 +30,8 @@ const int finalMask = externalMask << 1;
 
 const int staticMask = finalMask << 1;
 
+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;