Insert implicit downcasts in kernel strong mode.

pkg/kernel/lib/target/vm.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 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;
 import 'targets.dart';
 
 /// Specializes the kernel IR to the Dart VM.
 class VmTarget extends Target {
   final TargetFlags flags;
 
   VmTarget(this.flags);
@@ skipping 26 matching lines @@
         'dart:typed_data',
         'dart:vmservice_io',
         'dart:_vmservice',
         'dart:_builtin',
         'dart:nativewrappers',
         'dart:io',
       ];
 
   void transformProgram(Program program) {
     new mix.MixinFullResolution().transform(program);
+
+    if (strongMode) {
+      new InsertTypeChecks().transformProgram(program);
+    }
+
     cont.transformProgram(program);
 
     // Repair `_getMainClosure()` function in dart:_builtin.
     setup_builtin_library.transformProgram(program);
 
     if (strongMode) {
       new Erasure().transform(program);
     }
 
     new SanitizeForVM().transform(program);
   }
 }