Fix test after inferrer change.

tests/compiler/dart2js/list_tracer_test.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 'package:expect/expect.dart';
 import
     '../../../sdk/lib/_internal/compiler/implementation/types/types.dart'
     show ContainerTypeMask, TypeMask;
 
 import 'compiler_helper.dart';
@@ skipping 206 matching lines @@
   checkType('listSetInNonFinalField', typesInferrer.numType);
   checkType('listWithChangedLength', typesInferrer.intType.nullable());
 
   checkType('listPassedToClosure', typesInferrer.dynamicType);
   checkType('listReturnedFromClosure', typesInferrer.dynamicType);
   checkType('listUsedWithNonOkSelector', typesInferrer.dynamicType);
   checkType('listPassedAsOptionalParameter', typesInferrer.dynamicType);
   checkType('listPassedAsNamedParameter', typesInferrer.dynamicType);
 
   if (!allocation.contains('filled')) {
-    checkType('listUnset', new TypeMask.empty());
-    checkType('listOnlySetWithConstraint', new TypeMask.empty());
+    checkType('listUnset', new TypeMask.nonNullEmpty());
+    checkType('listOnlySetWithConstraint', new TypeMask.nonNullEmpty());
   }
 }