Insert implicit downcasts in kernel strong mode.

pkg/kernel/lib/target/vm.dart

Index: pkg/kernel/lib/target/vm.dart
diff --git a/pkg/kernel/lib/target/vm.dart b/pkg/kernel/lib/target/vm.dart
index e6c5d949a92f8fbec9c3453016582237fbddef08..855857c071fa2352dd75f07a01892f2bce0a898b 100644
--- a/pkg/kernel/lib/target/vm.dart
+++ b/pkg/kernel/lib/target/vm.dart
@@ -6,6 +6,7 @@ library kernel.target.vm;
 import '../ast.dart';
 import '../transformations/continuation.dart' as cont;
 import '../transformations/erasure.dart';
+import '../transformations/insert_type_checks.dart';
 import '../transformations/mixin_full_resolution.dart' as mix;
 import '../transformations/sanitize_for_vm.dart';
 import '../transformations/setup_builtin_library.dart' as setup_builtin_library;
@@ -52,6 +53,11 @@ class VmTarget extends Target {
 
   void transformProgram(Program program) {
     new mix.MixinFullResolution().transform(program);
+
+    if (strongMode) {
+      new InsertTypeChecks().transformProgram(program);
+    }
+
     cont.transformProgram(program);
 
     // Repair `_getMainClosure()` function in dart:_builtin.