Move kernel baseline tests to front_end.

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

 library;
 import self as self;
 import "dart:core" as core;
 
 class Mixin extends core::Object {
   field dynamic field;
   constructor •() → self::Mixin
     : super core::Object::•();
 }
 class A extends self::Object+Mixin+Mixin#0 {
   constructor •() → self::A
     : super self::Object+Mixin+Mixin#0::•();
 }
 abstract class Object+Mixin+Mixin#0 extends self::Object+Mixin#0 implements self::Mixin {
   field dynamic field;
   constructor •() → dynamic
     : super self::Object+Mixin#0::•();
 }
 abstract class Object+Mixin#0 extends core::Object implements self::Mixin {
   field dynamic field;
   constructor •() → dynamic
     : super core::Object::•();
 }