Move kernel baseline tests to front_end.

pkg/front_end/testcases/mixin.dart.outline.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class B extends self::Object&M1&M2 {
   constructor •(dynamic value) → void
     ;
 }
 abstract class M1 extends core::Object {
   constructor •() → void
@@ skipping 22 matching lines @@
     ;
 }
 abstract class Object&M1 = core::Object with self::M1 {
 }
 abstract class Object&M1&M2 = self::Object&M1 with self::M2 {
 }
 abstract class D^G1<S extends core::Object> = core::Object with self::G1<self::D^G1::S> {
 }
 static method main() → dynamic
   ;