Move kernel baseline tests to front_end.

pkg/kernel/testcases/input/prefer_baseclass.dart

-class A {}
-
-class B {}
-
-class AB1 extends A implements B {}
-
-class AB2 extends A implements B {}
-
-class BA1 extends B implements A {}
-
-class BA2 extends B implements A {}
-
-takeSubclassOfA(obj) {
-  // The analysis should at least infer that 'obj' is a subclass of A,
-  // When the upper bound is ambiguous, it should use the common superclass, if
-  // there is one besides Object.
-}
-
-takeSubclassOfB(obj) {
-  // Likewise, the analysis should infer that 'obj' is a subclass of B.
-}
-
-main() {
-  takeSubclassOfA(new AB1());
-  takeSubclassOfA(new AB2());
-
-  takeSubclassOfB(new BA1());
-  takeSubclassOfB(new BA2());
-}