Move TypeMask.simplify to a test helper.

tests/compiler/dart2js/simple_inferrer_final_field_test.dart

Index: tests/compiler/dart2js/simple_inferrer_final_field_test.dart
diff --git a/tests/compiler/dart2js/simple_inferrer_final_field_test.dart b/tests/compiler/dart2js/simple_inferrer_final_field_test.dart
index 15f77aba2159a2133ec482c9325635c165dbba6a..29d0940381aac6a011766763854a015f92cbab23 100644
--- a/tests/compiler/dart2js/simple_inferrer_final_field_test.dart
+++ b/tests/compiler/dart2js/simple_inferrer_final_field_test.dart
@@ -6,6 +6,7 @@ import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 import 'compiler_helper.dart';
 import 'parser_helper.dart';
+import 'type_mask_test_helper.dart';
 
 const String TEST = """
 
@@ -36,7 +37,7 @@ void main() {
       var cls = findElement(compiler, className);
       var element = cls.lookupLocalMember(fieldName);
       Expect.equals(type,
-          typesInferrer.getTypeOfElement(element).simplify(compiler));
+          simplify(typesInferrer.getTypeOfElement(element), compiler));
     }
 
     checkFieldTypeInClass('A', 'intField', compiler.typesTask.uint31Type);