Rewrite mixin application handling in Fasta.

pkg/front_end/testcases/rasta/issue_000048.dart.direct.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class A extends core::Object {
   field core::bool v1;
   field core::num v2;
   constructor •(core::bool v1, core::num v2) → void
     : self::A::v1 = v1, self::A::v2 = v2, super core::Object::•()
     ;
 }
 class M1 extends core::Object {
   field core::num v2 = 0;
   constructor •() → void
     : super core::Object::•()
     ;
 }
-class C extends self::A&M1 {
+class C extends self::A implements self::M1 {
+  field core::num v2 = 0;
   constructor •(core::bool v1, core::num v2) → void
-    : super self::A&M1::•(v1, v2)
-    ;
-}
-abstract class A&M1 extends self::A implements self::M1 {
-  field core::num v2 = 0;
-  constructor •(final core::bool v1, final core::num v2) → void
     : super self::A::•(v1, v2)
     ;
 }
 static method main() → dynamic {
   self::C c = new self::C::•(true, 2);
 }