Avoid dart:io dependency in try

tests/try/poi/diff_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.
 
 /// Test of element diff.
 library trydart.diff_test;
 
 import 'dart:async' show
     Future;
 
@@ skipping 13 matching lines @@
     Element,
     LibraryElement;
 
 import 'package:compiler/src/script.dart' show
     Script;
 
 import 'package:dart2js_incremental/diff.dart' show
     Difference,
     computeDifference;
 
-import '../../compiler/dart2js/compiler_helper.dart' show
+import '../../compiler/dart2js/mock_compiler.dart' show
     MockCompiler,
     compilerFor;
 
 final TEST_DATA = [
     {
       'beforeSource': 'main() {}',
       'afterSource': 'main() { var x; }',
       'expectations': [['main', 'main']],
     },
     {
@@ skipping 72 matching lines @@
         expectedBeforeName, elementNameOrNull(difference.before));
     Expect.stringEquals(expectedAfterName, elementNameOrNull(difference.after));
     print(difference);
   }
   Expect.isFalse(iterator.moveNext());
 }
 
 void main() {
   asyncTest(() => Future.forEach(TEST_DATA, testData));
 }