Insert covariance checks in strong mode.

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

Index: pkg/kernel/testcases/strong-mode/optional.baseline.txt
diff --git a/pkg/kernel/testcases/strong-mode/optional.baseline.txt b/pkg/kernel/testcases/strong-mode/optional.baseline.txt
index 2b911496eb2ba14e294843341046a6b325d6fec4..0ba01b938c5dc578e3be6797aeea92383f3d1e24 100644
--- a/pkg/kernel/testcases/strong-mode/optional.baseline.txt
+++ b/pkg/kernel/testcases/strong-mode/optional.baseline.txt
@@ -1,4 +1,3 @@
-// #lib1::InvalidListener is not a subtype of #lib1::Listener (optional.dart:42:21)
 // Too few positional arguments (optional.dart:44:22)
 // Too many positional arguments (optional.dart:45:22)
 // Too few positional arguments (optional.dart:46:25)
@@ -58,7 +57,7 @@ static method main() → dynamic {
   core::String string6 = extern.{self::External::externalMethod}(1, 2, 3);
   extern.{self::External::listen}(new self::TestListener::•());
   extern.{self::External::listen}(new self::ExtendedListener::•());
-  extern.{self::External::listen}(new self::InvalidListener::•());
+  extern.{self::External::listen}(new self::InvalidListener::•() as self::Listener);
   dynamic nothing1 = foo.{self::Foo::method}();
   dynamic nothing2 = foo.{self::Foo::method}(1, 2, 3, 4);
   core::String nothing3 = extern.{self::External::externalMethod}();