Move kernel baseline tests to front_end.

pkg/kernel/testcases/strong-mode/closure.baseline.txt

-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};
-}