Void is not required to be `null` anymore.

pkg/compiler/lib/src/inferrer/type_system.dart

 // Copyright (c) 2017, 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 '../elements/elements.dart';
 import '../elements/entities.dart';
 import '../elements/resolution_types.dart'
     show ResolutionDartType, ResolutionInterfaceType;
 import '../tree/dartstring.dart';
@@ skipping 264 matching lines @@
 
   /**
    * Returns the intersection between [type] and [annotation].
    * [isNullable] indicates whether the annotation implies a null
    * type.
    */
   TypeInformation narrowType(
       TypeInformation type, ResolutionDartType annotation,
       {bool isNullable: true}) {
     if (annotation.treatAsDynamic) return type;
-    if (annotation.isVoid) return nullType;
+    if (annotation.isVoid) return type;
     if (annotation.element == closedWorld.commonElements.objectClass &&
         isNullable) {
       return type;
     }
     TypeMask otherType;
     if (annotation.isTypedef || annotation.isFunctionType) {
       otherType = functionType.type;
     } else if (annotation.isTypeVariable) {
       // TODO(ngeoffray): Narrow to bound.
       return type;
@@ skipping 256 matching lines @@
     TypeMask newType = null;
     for (TypeMask mask in list) {
       newType = newType == null ? mask : newType.union(mask, closedWorld);
       // Likewise - stop early if we already reach dynamic.
       if (newType.containsAll(closedWorld)) return dynamicType;
     }
 
     return newType ?? const TypeMask.nonNullEmpty();
   }
 }