Rewrite mixin application handling in Fasta.

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

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