Move TypeMask.simplify to a test helper.

tests/compiler/dart2js/cpa_inference_test.dart

 // Copyright (c) 2012, 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 'dart:async';
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 import '../../../sdk/lib/_internal/compiler/implementation/source_file.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/types/types.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/inferrer/concrete_types_inferrer.dart';
 
 import "parser_helper.dart";
 import "compiler_helper.dart";
+import "type_mask_test_helper.dart";
 import 'dart:mirrors';
 
 /**
  * Finds the node corresponding to the last occurence of the substring
  * [: identifier; :] in the program represented by the visited AST.
  */
 class VariableFinderVisitor extends Visitor {
   final String identifier;
   Node result;
 
@@ skipping 1540 matching lines @@
     }
 
     Expect.equals(convert(singleton(a).union(singleton(b))),
                   new TypeMask.nonNullSubclass(a));
 
     Expect.equals(
         convert(singleton(a).union(singleton(b)).union(nullSingleton)),
                   new TypeMask.subclass(a));
 
     Expect.equals(
-        convert(singleton(b).union(singleton(d))).simplify(result.compiler),
+        simplify(convert(singleton(b).union(singleton(d))), result.compiler),
         new TypeMask.nonNullSubtype(a));
   });
 }
 
 testSelectors() {
   final String source = r"""
       // ABC <--- A
       //       `- BC <--- B
       //               `- C
 
@@ skipping 512 matching lines @@
     testClosures10,
     testClosures11,
     testClosures12,
     testRefinement,
     testDefaultArguments,
     testSuperConstructorCall,
     testSuperConstructorCall2,
     testSuperConstructorCall3,
   ], (f) => f()));
 }