Move kernel baseline tests to front_end.

pkg/kernel/testcases/input/optional.dart

-class Foo {
-  method(x, [y, z]) {
-    return "string";
-  }
-}
-
-abstract class External {
-  String externalMethod(int x, [int y, int z]);
-  void listen(Listener listener);
-}
-
-external External createExternal();
-
-abstract class Listener {
-  void event(String input, [int x, int y]);
-}
-
-class TestListener extends Listener {
-  void event(input, [x, y]) {}
-}
-
-class ExtendedListener extends Listener {
-  void event(input, [x, y, z]) {}
-}
-
-class InvalidListener {
-  void event(input, [x]) {}
-}
-
-main() {
-  var foo = new Foo();
-  var string1 = foo.method(1);
-  var string2 = foo.method(1, 2);
-  var string3 = foo.method(1, 2, 3);
-
-  var extern = createExternal();
-  var string4 = extern.externalMethod(1);
-  var string5 = extern.externalMethod(1, 2);
-  var string6 = extern.externalMethod(1, 2, 3);
-
-  extern.listen(new TestListener());
-  extern.listen(new ExtendedListener());
-  extern.listen(new InvalidListener());
-
-  var nothing1 = foo.method();
-  var nothing2 = foo.method(1, 2, 3, 4);
-  var nothing3 = extern.externalMethod();
-  var nothing4 = extern.externalMethod(1, 2, 3, 4);
-}