Rewrite mixin application handling in Fasta.

pkg/front_end/testcases/rasta/class_hierarchy.dart.outline.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class A extends core::Object {
   constructor •() → void
     ;
 }
 class B extends core::Object {
   constructor •() → void
     ;
 }
-class C extends self::Object&Missing {
+class C extends core::Object {
   constructor •() → void
     : super core::Object::•()
     ;
 }
 class D extends core::Object {
   static field dynamic _redirecting# = <dynamic>[self::D::•];
   static factory •() → dynamic
     throw "Redirection constructor target not found: Missing";
 }
-abstract class Object&Missing extends core::Object {
-  constructor •() → void
-    ;
-}
 static method main() → void
   ;