Move TypeMask.simplify to a test helper.

tests/compiler/dart2js/list_tracer_test.dart

Index: tests/compiler/dart2js/list_tracer_test.dart
diff --git a/tests/compiler/dart2js/list_tracer_test.dart b/tests/compiler/dart2js/list_tracer_test.dart
index e2e8d828cf9b60a97ea85de3aa33cc3e28c2f062..319cfddf4cc1955d2d3996994aa54ea3e41604cc 100644
--- a/tests/compiler/dart2js/list_tracer_test.dart
+++ b/tests/compiler/dart2js/list_tracer_test.dart
@@ -10,7 +10,7 @@ import
 
 import 'compiler_helper.dart';
 import 'parser_helper.dart';
-
+import 'type_mask_test_helper.dart';
 
 String generateTest(String listAllocation) {
   return """
@@ -208,7 +208,7 @@ void doTest(String allocation, {bool nullify}) {
       var element = findElement(compiler, name);
       ContainerTypeMask mask = typesInferrer.getTypeOfElement(element);
       if (nullify) type = type.nullable();
-      Expect.equals(type, mask.elementType.simplify(compiler), name);
+      Expect.equals(type, simplify(mask.elementType, compiler), name);
     }
 
     checkType('listInField', typesTask.numType);