Rewrite mixin application handling in Fasta.

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

Index: pkg/front_end/testcases/rasta/duplicated_mixin.dart.outline.expect
diff --git a/pkg/front_end/testcases/rasta/duplicated_mixin.dart.outline.expect b/pkg/front_end/testcases/rasta/duplicated_mixin.dart.outline.expect
index eb523ab3024273ac6b1994e84e18d4a369205d36..1c4d84df864f2b3c879d48a78cb83c40401218d9 100644
--- a/pkg/front_end/testcases/rasta/duplicated_mixin.dart.outline.expect
+++ b/pkg/front_end/testcases/rasta/duplicated_mixin.dart.outline.expect
@@ -7,11 +7,11 @@ class Mixin extends core::Object {
   constructor •() → void
     ;
 }
-class A extends self::Object&Mixin&Mixin {
-  constructor •() → void
-    ;
-}
 abstract class Object&Mixin = core::Object with self::Mixin {
 }
 abstract class Object&Mixin&Mixin = self::Object&Mixin with self::Mixin {
 }
+class A extends self::Object&Mixin&Mixin {
+  constructor •() → void
+    ;
+}