Move kernel baseline tests to front_end.

pkg/front_end/test/fasta/rasta/duplicated_mixin.dart.outline.expect

-library;
-import self as self;
-import "dart:core" as core;
-
-class Mixin extends core::Object {
-  field dynamic field;
-  constructor •() → void
-    ;
-}
-class A extends self::Object&Mixin&Mixin {
-  constructor •() → void
-    ;
-}
-abstract class Object&Mixin = core::Object with self::Mixin {
-}
-abstract class Object&Mixin&Mixin = self::Object&Mixin with self::Mixin {
-}