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

pkg/compiler/lib/src/ssa/codegen.dart

 // Copyright (c) 2012, 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 'dart:math' as math;
 import 'dart:collection' show Queue;
 import '../common.dart';
 import '../common/codegen.dart' show CodegenRegistry, CodegenWorkItem;
 import '../common/tasks.dart' show CompilerTask;
 import '../constants/constant_system.dart';
@@ skipping 2910 matching lines @@
       body = unwrapStatement(body);
       pushStatement(new js.If.noElse(test, body)
           .withSourceInformation(node.sourceInformation));
       return;
     }
 
     assert(node.isCheckedModeCheck || node.isCastTypeCheck);
     DartType type = node.typeExpression;
     assert(!type.isTypedef);
     assert(!type.isDynamic);
-    assert(!type.isVoid);
     if (type.isFunctionType) {
       // TODO(5022): We currently generate $isFunction checks for
       // function types.
       _registry
           .registerTypeUse(new TypeUse.isCheck(_commonElements.functionType));
     }
     _registry.registerTypeUse(new TypeUse.isCheck(type));
 
     CheckedModeHelper helper;
     if (node.isBooleanConversionCheck) {
@@ skipping 98 matching lines @@
     return _closedWorld.anyStrictSubclassOf(cls, (ClassEntity subclass) {
       return !_rtiSubstitutions.isTrivialSubstitution(subclass, cls);
     });
   }
 
   @override
   void visitRef(HRef node) {
     visit(node.value);
   }
 }