Rewrite mixin application handling in Fasta.

pkg/front_end/testcases/rasta/issue_000007.dart.outline.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class Base extends core::Object {
   constructor •() → void
     ;
 }
 class Mixin extends core::Object {
   constructor •() → void
     ;
   method foo() → dynamic
     ;
 }
+abstract class Base&Mixin = self::Base with self::Mixin {
+}
 class Sub extends self::Base&Mixin {
   constructor •() → void
     ;
 }
-abstract class Base&Mixin = self::Base with self::Mixin {
-}
 static method main() → dynamic
   ;