Remove warnings from dart2js test-suite.

tests/compiler/dart2js/bad_output_io_test.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.
 
 // Test that the compiler can handle imports when package root has not been set.
 
 library dart2js.test.missing_file;
 
 import 'dart:io' show exit;
 import 'package:expect/expect.dart';
-import "package:async_helper/async_helper.dart";
 
 import '../../../sdk/lib/_internal/compiler/compiler.dart'
        show Diagnostic;
 import '../../../sdk/lib/_internal/compiler/implementation/dart2js.dart'
        show exitFunc, compileFunc, compile, diagnosticHandler;
 import '../../../sdk/lib/_internal/compiler/implementation/source_file_provider.dart'
        show FormattingDiagnosticHandler;
 
 class CollectingFormattingDiagnosticHandler
     implements FormattingDiagnosticHandler {
@@ skipping 16 matching lines @@
    messages.add([message, kind]);
   }
 
   void diagnosticHandler(Uri uri, int begin, int end, String message, kind) {
     messages.add([message, kind]);
   }
 
   void call(Uri uri, int begin, int end, String message, kind) {
     diagnosticHandler(uri, begin, end, message, kind);
   }
+
+  String prefixMessage(String message, Diagnostic kind) {
+    return message;
+  }
 }
 
 testOutputProvider(script, libraryRoot, packageRoot, inputProvider, handler,
                    [options, outputProvider, environment]) {
   diagnosticHandler = new CollectingFormattingDiagnosticHandler();
   outputProvider("/non/existing/directory/should/fail/file", "js");
 }
 
 void main() {
   compileFunc = testOutputProvider;
   exitFunc = (exitCode) {
-    Expect.equals(1, diagnosticHandler.messages.length);
-    var message = diagnosticHandler.messages[0];
+    CollectingFormattingDiagnosticHandler handler = diagnosticHandler;
+    Expect.equals(1, handler.messages.length);
+    var message = handler.messages[0];
     Expect.isTrue(message[0].contains("Cannot open file"));
     Expect.equals(Diagnostic.ERROR, message[1]);
     Expect.equals(1, exitCode);
     exit(0);
   };
   compile(["foo.dart", "--out=bar.dart"]);
 }