Move kernel baseline tests to front_end.

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

 library;
 import self as self;
 import "dart:core" as core;
 
 class Base extends core::Object {
   constructor •() → self::Base
     : super core::Object::•();
 }
 class Mixin extends core::Object {
   constructor •() → self::Mixin
     : super core::Object::•();
   method foo() → dynamic
     return core::print("foo");
 }
 class Sub extends self::Base+Mixin#0 {
   constructor •() → self::Sub
     : super self::Base+Mixin#0::•();
 }
 abstract class Base+Mixin#0 extends self::Base implements self::Mixin {
   constructor •() → self::Base
     : super self::Base::•();
   method foo() → dynamic
     return core::print("foo");
 }
 static method main() → dynamic {
   new self::Sub::•().foo();
 }