Move TypeMask.simplify to a test helper.

tests/compiler/dart2js/simple_inferrer_const_closure4_test.dart

 // Copyright (c) 2014, 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 'type_mask_test_helper.dart';
 
 const String TEST = """
 
 method(a) {  // Called via [foo] with integer then double.
   return a;
 }
 
 const foo = method;
 
 returnNum(x) {
@@ skipping 11 matching lines @@
   Uri uri = new Uri(scheme: 'source');
   var compiler = compilerFor(TEST, uri);
   asyncTest(() => compiler.runCompiler(uri).then((_) {
     var typesInferrer = compiler.typesTask.typesInferrer;
 
     checkArgument(String functionName, type) {
       var functionElement = findElement(compiler, functionName);
       var signature = functionElement.functionSignature;
       var element = signature.requiredParameters.first;
       Expect.equals(type,
-          typesInferrer.getTypeOfElement(element).simplify(compiler),
+          simplify(typesInferrer.getTypeOfElement(element), compiler),
           functionName);
     }
 
     checkArgument('method', compiler.typesTask.numType);
     checkArgument('returnNum', compiler.typesTask.numType);
   }));
 }