Move TypeMask.simplify to a test helper.

tests/compiler/dart2js/list_tracer2_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.
 
 // We used to always nullify the element type of a list we are tracing in
 // the presence of a fixed length list constructor call.
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 import
     '../../../sdk/lib/_internal/compiler/implementation/types/types.dart'
     show ContainerTypeMask, TypeMask;
 
 import 'compiler_helper.dart';
 import 'parser_helper.dart';
-
+import 'type_mask_test_helper.dart';
 
 const String TEST = r'''
 var myList = [42];
 main() {
   var a = new List(42);
   return myList[0];
 }
 ''';
 
 void main() {
   Uri uri = new Uri(scheme: 'source');
   var compiler = compilerFor(TEST, uri);
   asyncTest(() => compiler.runCompiler(uri).then((_) {
     var typesInferrer = compiler.typesTask.typesInferrer;
 
     checkType(String name, type) {
       var element = findElement(compiler, name);
       ContainerTypeMask mask = typesInferrer.getTypeOfElement(element);
-      Expect.equals(type, mask.elementType.simplify(compiler), name);
+      Expect.equals(type, simplify(mask.elementType, compiler), name);
     }
 
     checkType('myList', compiler.typesTask.uint31Type);
   }));
 }