Move TypeMask.simplify to a test helper.

tests/compiler/dart2js/closure_tracer_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 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 = '''
 testFunctionStatement() {
   var res;
   closure(a) => res = a;
   closure(42);
   return res;
 }
 
 testFunctionExpression() {
@@ skipping 89 matching lines @@
 void main() {
   Uri uri = new Uri(scheme: 'source');
   var compiler = compilerFor(TEST, uri);
   asyncTest(() => compiler.runCompiler(uri).then((_) {
     var typesTask = compiler.typesTask;
     var typesInferrer = typesTask.typesInferrer;
 
     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);
     checkType('testFunctionExpression', typesTask.uint31Type);
     checkType('testStoredInInstance', typesTask.uint31Type);
     checkType('testStoredInStatic', typesTask.uint31Type);
     checkType('testPassedInParameter', typesTask.uint31Type);
     checkType('testStaticClosure1', typesTask.uint31Type);
     checkType('testStaticClosure2', typesTask.numType);
     checkType('testStaticClosure3', typesTask.uint31Type);
     checkType('testStaticClosure4', typesTask.numType);
   }));
 }