Insert implicit downcasts in kernel strong mode.

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

Index: pkg/kernel/testcases/strong-mode/call.baseline.txt
diff --git a/pkg/kernel/testcases/strong-mode/call.baseline.txt b/pkg/kernel/testcases/strong-mode/call.baseline.txt
index 387757ec86fbd0d849de32e3331b60e31896f07f..65b056156c2c0322412354cc3f8486a281433284 100644
--- a/pkg/kernel/testcases/strong-mode/call.baseline.txt
+++ b/pkg/kernel/testcases/strong-mode/call.baseline.txt
@@ -1,8 +1,8 @@
-// Too few positional arguments (call.dart:30:17)
-// Too few positional arguments (call.dart:31:25)
-// Too few positional arguments (call.dart:35:17)
-// Too few positional arguments (call.dart:36:26)
-// Too few positional arguments (call.dart:37:31)
+// Too few positional arguments (call.dart:30:18)
+// Too few positional arguments (call.dart:31:26)
+// Too few positional arguments (call.dart:35:18)
+// Too few positional arguments (call.dart:36:27)
+// Too few positional arguments (call.dart:37:32)
 library;
 import self as self;
 import "dart:core" as core;