Move kernel baseline tests to front_end.

pkg/front_end/testcases/closure.dart.strong.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class Foo extends core::Object {
   field self::Bar _field = new self::Bar::•();
   constructor •() → void
     : super core::Object::•()
     ;
 }
 class Bar extends core::Object {
   constructor •() → void
     : super core::Object::•()
     ;
 }
 static method useCallback(dynamic callback) → dynamic {
   dynamic _ = callback.call();
 }
 static method main() → dynamic {
   dynamic x;
   function inner() → dynamic {
     x = new self::Foo::•();
     return new self::Foo::•();
   }
   self::useCallback(inner);
   self::Bar _ = inner.call().{self::Foo::_field};
 }