Move kernel baseline tests to front_end.

pkg/front_end/testcases/optional.dart.outline.expect

+library;
+import self as self;
+import "dart:core" as core;
+
+class Foo extends core::Object {
+  constructor •() → void
+    ;
+  method method(dynamic x, [dynamic y, dynamic z]) → dynamic
+    ;
+}
+abstract class External extends core::Object {
+  constructor •() → void
+    ;
+  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
+    ;
+  abstract method event(core::String input, [core::int x, core::int y]) → void;
+}
+class TestListener extends self::Listener {
+  constructor •() → void
+    ;
+  method event(dynamic input, [dynamic x, dynamic y]) → void
+    ;
+}
+class ExtendedListener extends self::Listener {
+  constructor •() → void
+    ;
+  method event(dynamic input, [dynamic x, dynamic y, dynamic z]) → void
+    ;
+}
+class InvalidListener extends core::Object {
+  constructor •() → void
+    ;
+  method event(dynamic input, [dynamic x]) → void
+    ;
+}
+external static abstract method createExternal() → self::External;
+static method main() → dynamic
+  ;