Avoid dart:io dependency in try

tests/try/poi/compiler_test_case.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.
 
 // Helper class for writing compiler tests.
 library trydart.compiler_test_case;
 
 import 'dart:async' show
     Future;
 
 export 'dart:async' show
     Future;
 
 import 'package:async_helper/async_helper.dart' show
     asyncTest;
 
-import '../../compiler/dart2js/compiler_helper.dart' show
+import '../../compiler/dart2js/mock_compiler.dart' show
     MockCompiler,
     compilerFor;
 
 export 'package:expect/expect.dart' show
     Expect;
 
 import 'package:compiler/src/elements/elements.dart' show
     LibraryElement;
 
 export 'package:compiler/src/elements/elements.dart' show
@@ skipping 49 matching lines @@
 
   /// Returns a future for the mainApp after running the compiler.
   Future<LibraryElement> compile() {
     return loadMainApp().then((LibraryElement library) {
       return compiler.runCompiler(scriptUri).then((_) => library);
     });
   }
 
   String toString() => source;
 }