Rewrite mixin application handling in Fasta.

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

 library;
 import self as self;
 import "dart:core" as core;
 
 abstract class FooBase<Tf extends core::Object> extends core::Object {
   static field dynamic _redirecting# = <dynamic>[self::FooBase::•];
   abstract get x() → core::int;
   static factory •<Tf extends core::Object>(core::int x) → dynamic
     let final dynamic #t1 = self::Foo::• in invalid-expression;
 }
@@ skipping 28 matching lines @@
     ;
 }
 class Base<M extends core::Object> extends core::Object {
   constructor •() → void
     ;
 }
 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
   ;