Move TypeMask.simplify to a test helper.

tests/compiler/dart2js/simple_inferrer_postfix_prefix_test.dart

Index: tests/compiler/dart2js/simple_inferrer_postfix_prefix_test.dart
diff --git a/tests/compiler/dart2js/simple_inferrer_postfix_prefix_test.dart b/tests/compiler/dart2js/simple_inferrer_postfix_prefix_test.dart
index 0b1e8b043d873d4f19371291ef01e9a93d6a2834..20503548b95c9d8b375dd0c7e035512297d38808 100644
--- a/tests/compiler/dart2js/simple_inferrer_postfix_prefix_test.dart
+++ b/tests/compiler/dart2js/simple_inferrer_postfix_prefix_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 = """
 
@@ -72,7 +73,7 @@ void main() {
       var cls = findElement(compiler, className);
       var element = cls.lookupLocalMember(methodName);
       Expect.equals(type,
-          typesInferrer.getReturnTypeOfElement(element).simplify(compiler),
+          simplify(typesInferrer.getReturnTypeOfElement(element), compiler),
           methodName);
     }