Insert implicit downcasts in kernel 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 1087a0382c4aac282bb08f426667461764413fb2..2b911496eb2ba14e294843341046a6b325d6fec4 100644
--- a/pkg/kernel/testcases/strong-mode/optional.baseline.txt
+++ b/pkg/kernel/testcases/strong-mode/optional.baseline.txt
@@ -1,8 +1,8 @@
-// #lib1::InvalidListener is not a subtype of #lib1::Listener (optional.dart:42:20)
-// Too few positional arguments (optional.dart:44:21)
-// Too many positional arguments (optional.dart:45:21)
-// Too few positional arguments (optional.dart:46:24)
-// Too many positional arguments (optional.dart:47:24)
+// #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)
+// Too many positional arguments (optional.dart:47:25)
 library;
 import self as self;
 import "dart:core" as core;