Revert "Void is not required to be `null` anymore."

pkg/compiler/lib/src/js_backend/checked_mode_helpers.dart

 // Copyright (c) 2013, 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.
 
 import '../common.dart';
 import '../common_elements.dart';
 import '../elements/elements.dart' show ErroneousElement;
 import '../elements/entities.dart';
 import '../elements/resolution_types.dart' show MalformedType;
 import '../elements/types.dart';
@@ skipping 99 matching lines @@
 }
 
 class CheckedModeHelpers {
   final CommonElements _commonElements;
 
   CheckedModeHelpers(this._commonElements);
 
   /// All the checked mode helpers.
   static const List<CheckedModeHelper> helpers = const <CheckedModeHelper>[
     const MalformedCheckedModeHelper('checkMalformedType'),
+    const CheckedModeHelper('voidTypeCheck'),
     const CheckedModeHelper('stringTypeCast'),
     const CheckedModeHelper('stringTypeCheck'),
     const CheckedModeHelper('doubleTypeCast'),
     const CheckedModeHelper('doubleTypeCheck'),
     const CheckedModeHelper('numTypeCast'),
     const CheckedModeHelper('numTypeCheck'),
     const CheckedModeHelper('boolTypeCast'),
     const CheckedModeHelper('boolTypeCheck'),
     const CheckedModeHelper('intTypeCast'),
     const CheckedModeHelper('intTypeCheck'),
@@ skipping 65 matching lines @@
 
   String getCheckedModeHelperNameInternal(DartType type,
       {bool typeCast, bool nativeCheckOnly}) {
     assert(!type.isTypedef);
     if (type.isMalformed) {
       // The same error is thrown for type test and type cast of a malformed
       // type so we only need one check method.
       return 'checkMalformedType';
     }
 
+    if (type.isVoid) {
+      assert(!typeCast); // Cannot cast to void.
+      if (nativeCheckOnly) return null;
+      return 'voidTypeCheck';
+    }
+
     if (type.isTypeVariable) {
       return typeCast
           ? 'subtypeOfRuntimeTypeCast'
           : 'assertSubtypeOfRuntimeType';
     }
 
     if (type.isFunctionType) {
       return typeCast ? 'functionTypeCast' : 'functionTypeCheck';
     }
 
@@ skipping 68 matching lines @@
 
     if (nativeCheck) {
       // TODO(karlklose): can we get rid of this branch when we use
       // interceptors?
       return 'intercepted$suffix';
     } else {
       return 'property$suffix';
     }
   }
 }