Move TypeMask.simplify to a test helper.

tests/compiler/dart2js/closure_tracer_test.dart

Index: tests/compiler/dart2js/closure_tracer_test.dart
diff --git a/tests/compiler/dart2js/closure_tracer_test.dart b/tests/compiler/dart2js/closure_tracer_test.dart
index 600f228bc36d11468972de6d686b6f674de8ee81..af72fe0a26ac85791d5be0695d0c6aacc71ded49 100644
--- a/tests/compiler/dart2js/closure_tracer_test.dart
+++ b/tests/compiler/dart2js/closure_tracer_test.dart
@@ -7,6 +7,7 @@ import "package:async_helper/async_helper.dart";
 
 import 'compiler_helper.dart';
 import 'parser_helper.dart';
+import 'type_mask_test_helper.dart';
 
 const String TEST = '''
 testFunctionStatement() {
@@ -116,7 +117,7 @@ void main() {
     checkType(String name, type) {
       var element = findElement(compiler, name);
       var mask = typesInferrer.getReturnTypeOfElement(element);
-      Expect.equals(type.nullable(), mask.simplify(compiler), name);
+      Expect.equals(type.nullable(), simplify(mask, compiler), name);
     }
 
     checkType('testFunctionStatement', typesTask.uint31Type);