Rewrite mixin application handling in Fasta.

pkg/front_end/testcases/redirecting_factory.dart.outline.expect

Index: pkg/front_end/testcases/redirecting_factory.dart.outline.expect
diff --git a/pkg/front_end/testcases/redirecting_factory.dart.outline.expect b/pkg/front_end/testcases/redirecting_factory.dart.outline.expect
index cf4e8e262e0d5af3800b9806651c7260561abc32..bd411a5aab469f3a5c74cf009d651350dd57c426 100644
--- a/pkg/front_end/testcases/redirecting_factory.dart.outline.expect
+++ b/pkg/front_end/testcases/redirecting_factory.dart.outline.expect
@@ -46,12 +46,10 @@ class Mixin<M extends core::Object> extends core::Object {
   constructor •() → void
     ;
 }
-class Mix<M extends core::Object> extends self::Mix^Mixin<self::Mix::M> {
+class Mix<M extends core::Object> = self::Base<self::Mix::M> with self::Mixin<self::Mix::M> {
   constructor •() → void
     : super self::Base::•()
     ;
 }
-abstract class Mix^Mixin<M extends core::Object> = self::Base<self::Mix^Mixin::M> with self::Mixin<self::Mix^Mixin::M> {
-}
 static method main() → dynamic
   ;