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

pkg/compiler/lib/src/js_backend/impact_transformer.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.
 
 library js_backend.backend.impact_transformer;
 
 import '../common.dart';
 import '../common_elements.dart';
 import '../common/backend_api.dart' show ImpactTransformer;
 import '../common/codegen.dart' show CodegenImpact;
@@ skipping 345 matching lines @@
       this._backendUsage,
       this._rtiNeed,
       this._nativeCodegenEnqueuer,
       this._namer,
       this._oneShotInterceptorData,
       this._lookupMapAnalysis,
       this._rtiChecksBuilder);
 
   void onIsCheckForCodegen(DartType type, TransformedWorldImpact transformed) {
     if (type.isDynamic) return;
-    if (type.isVoid) return;
     type = type.unaliased;
     _impacts.typeCheck.registerImpact(transformed, _elementEnvironment);
 
     bool inCheckedMode = _options.enableTypeAssertions;
     // [registerIsCheck] is also called for checked mode checks, so we
     // need to register checked mode helpers.
     if (inCheckedMode) {
       // All helpers are added to resolution queue in enqueueHelpers. These
       // calls to [enqueue] with the resolution enqueuer serve as assertions
       // that the helper was in fact added.
@@ skipping 90 matching lines @@
           _impacts.asyncStarBody
               .registerImpact(transformed, _elementEnvironment);
           break;
       }
     }
 
     // TODO(johnniwinther): Remove eager registration.
     return transformed;
   }
 }