Avoid dart:io dependency in try

tests/try/poi/forget_element_assertion.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.
 
 // Tests that dart2js doesn't support metadata on local elements.
 // Remove this file when dart2js support such features.
 library trydart.forget_element_assertion;
 
-import '../../compiler/dart2js/compiler_helper.dart' show
+import '../../compiler/dart2js/mock_compiler.dart' show
     compilerFor;
 
 import 'compiler_test_case.dart';
 
 class FailingTestCase extends CompilerTestCase {
   FailingTestCase.intermediate(String source, int errorCount, Uri scriptUri)
       : super.init(
           source, scriptUri,
           compilerFor(source, scriptUri, expectedErrors: errorCount));
 
@@ skipping 35 matching lines @@
       // a positive test in forget_element_test.dart. For example:
       //
       //    new ForgetElementTestCase(
       //       'main() { @Constant() var x; return x; } $CONSTANT_CLASS',
       //       metadataCount: 1),
       new FailingTestCase(
           'main() { @Constant() var x; return x; } $CONSTANT_CLASS',
           1),
     ];
 }