Move kernel baseline tests to front_end.

pkg/kernel/testcases/spec-mode/optional.baseline.txt

-library;
-import self as self;
-import "dart:core" as core;
-
-class Foo extends core::Object {
-  constructor •() → void
-    : super core::Object::•()
-    ;
-  method method(dynamic x, [dynamic y, dynamic z]) → dynamic {
-    return "string";
-  }
-}
-abstract class External extends core::Object {
-  constructor •() → void
-    : super core::Object::•()
-    ;
-  abstract method externalMethod(core::int x, [core::int y, core::int z]) → core::String;
-  abstract method listen(self::Listener listener) → void;
-}
-abstract class Listener extends core::Object {
-  constructor •() → void
-    : super core::Object::•()
-    ;
-  abstract method event(core::String input, [core::int x, core::int y]) → void;
-}
-class TestListener extends self::Listener {
-  constructor •() → void
-    : super self::Listener::•()
-    ;
-  method event(dynamic input, [dynamic x, dynamic y]) → void {}
-}
-class ExtendedListener extends self::Listener {
-  constructor •() → void
-    : super self::Listener::•()
-    ;
-  method event(dynamic input, [dynamic x, dynamic y, dynamic z]) → void {}
-}
-class InvalidListener extends core::Object {
-  constructor •() → void
-    : super core::Object::•()
-    ;
-  method event(dynamic input, [dynamic x]) → void {}
-}
-external static method createExternal() → self::External;
-static method main() → dynamic {
-  dynamic foo = new self::Foo::•();
-  dynamic string1 = foo.method(1);
-  dynamic string2 = foo.method(1, 2);
-  dynamic string3 = foo.method(1, 2, 3);
-  dynamic extern = self::createExternal();
-  dynamic string4 = extern.externalMethod(1);
-  dynamic string5 = extern.externalMethod(1, 2);
-  dynamic string6 = extern.externalMethod(1, 2, 3);
-  extern.listen(new self::TestListener::•());
-  extern.listen(new self::ExtendedListener::•());
-  extern.listen(new self::InvalidListener::•());
-  dynamic nothing1 = foo.method();
-  dynamic nothing2 = foo.method(1, 2, 3, 4);
-  dynamic nothing3 = extern.externalMethod();
-  dynamic nothing4 = extern.externalMethod(1, 2, 3, 4);
-}