Move TypeMask.simplify to a test helper.

tests/compiler/dart2js/list_tracer3_test.dart

Index: tests/compiler/dart2js/list_tracer3_test.dart
diff --git a/tests/compiler/dart2js/list_tracer3_test.dart b/tests/compiler/dart2js/list_tracer3_test.dart
index d8fb1773ab215baba65cc1278e117385998ad1e1..4cf3a8f6083b7422b538d7488a5ac4227c075343 100644
--- a/tests/compiler/dart2js/list_tracer3_test.dart
+++ b/tests/compiler/dart2js/list_tracer3_test.dart
@@ -13,6 +13,7 @@ import
 
 import 'compiler_helper.dart';
 import 'parser_helper.dart';
+import 'type_mask_test_helper.dart';
 
 
 const String TEST = r'''
@@ -34,7 +35,7 @@ void main() {
     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);
     }
 
     var interceptorType =