Remove warnings from dart2js test-suite.

tests/compiler/dart2js/memory_source_file_helper.dart

 // Copyright (c) 2013, 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.
 
 library dart2js.test.memory_source_file_helper;
 
 import 'dart:async' show Future;
-import 'dart:io';
 export 'dart:io' show Platform;
 
 export '../../../sdk/lib/_internal/compiler/implementation/apiimpl.dart'
        show Compiler;
 
 export '../../../sdk/lib/_internal/compiler/implementation/filenames.dart'
        show currentDirectory;
 
 import '../../../sdk/lib/_internal/compiler/implementation/source_file.dart'
        show StringSourceFile;
@@ skipping 17 matching lines @@
     if (source == null) {
       return new Future.error(new Exception('No such file $resourceUri'));
     }
     String resourceName = '$resourceUri';
     this.sourceFiles[resourceName] = new StringSourceFile(resourceName, source);
     return new Future.value(source);
   }
 
   Future<String> call(Uri resourceUri) => readStringFromUri(resourceUri);
 }