Insert implicit downcasts in kernel strong mode.

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

-// 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;
 
 class Callable extends core::Object {
   constructor •() → void
     : super core::Object::•()
     ;
   method call(dynamic x) → dynamic {
     return "string";
@@ skipping 28 matching lines @@
   dynamic nothing4 = closure.call.call.call();
   dynamic nothing5 = callable.{self::Callable::call}();
   dynamic nothing6 = callable.{self::Callable::call}();
   dynamic nothing7 = callable.{self::Callable::call}.call();
   dynamic nothing8 = callable.{self::Callable::call}.call.call();
   dynamic nothing9 = callableGetter.call();
   dynamic nothing10 = callableGetter.{self::CallableGetter::call}.call();
   dynamic nothing11 = callableGetter.{self::CallableGetter::call}.call();
   dynamic nothing12 = callableGetter.{self::CallableGetter::call}.call.call();
 }