Rewrite mixin application handling in Fasta.

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

Index: pkg/front_end/testcases/rasta/class_hierarchy.dart.outline.expect
diff --git a/pkg/front_end/testcases/rasta/class_hierarchy.dart.outline.expect b/pkg/front_end/testcases/rasta/class_hierarchy.dart.outline.expect
index f77dde6947884efb43d7ff77d0f959237046817c..90616a9dc8f46109198e52ca42c0feb3d62982d8 100644
--- a/pkg/front_end/testcases/rasta/class_hierarchy.dart.outline.expect
+++ b/pkg/front_end/testcases/rasta/class_hierarchy.dart.outline.expect
@@ -10,7 +10,7 @@ class B extends core::Object {
   constructor •() → void
     ;
 }
-class C extends self::Object&Missing {
+class C extends core::Object {
   constructor •() → void
     : super core::Object::•()
     ;
@@ -20,9 +20,5 @@ class D extends core::Object {
   static factory •() → dynamic
     throw "Redirection constructor target not found: Missing";
 }
-abstract class Object&Missing extends core::Object {
-  constructor •() → void
-    ;
-}
 static method main() → void
   ;