Move kernel baseline tests to front_end.

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

-library;
-import self as self;
-import "dart:core" as core;
-
-abstract class A extends core::Object {
-  constructor •() → void
-    : super core::Object::•()
-    ;
-}
-abstract class B extends core::Object {
-  constructor •() → void
-    : super core::Object::•()
-    ;
-}
-class AB1 extends self::A implements self::B {
-  constructor •() → void
-    : super self::A::•()
-    ;
-}
-class AB2 extends self::A implements self::B {
-  constructor •() → void
-    : super self::A::•()
-    ;
-}
-class BA1 extends self::B implements self::A {
-  constructor •() → void
-    : super self::B::•()
-    ;
-}
-class BA2 extends self::B implements self::A {
-  constructor •() → void
-    : super self::B::•()
-    ;
-}
-static method takeSubclassOfA(dynamic obj) → dynamic {}
-static method takeSubclassOfB(dynamic obj) → dynamic {}
-static method main() → dynamic {
-  self::takeSubclassOfA(new self::AB1::•());
-  self::takeSubclassOfA(new self::AB2::•());
-  self::takeSubclassOfB(new self::BA1::•());
-  self::takeSubclassOfB(new self::BA2::•());
-}