Replace Backend with JavaScriptBackend.

tests/compiler/dart2js/exit_code_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 the exit code of dart2js in case of exceptions, errors, warnings, etc.
 
 import 'dart:async';
 import 'dart:io' show Platform;
 
 import 'package:async_helper/async_helper.dart';
@@ skipping 36 matching lines @@
       String this.testType,
       Function this.onTest)
       : reporter = new TestDiagnosticReporter(),
         super(inputProvider, outputProvider, handler, options) {
     reporter.compiler = this;
     reporter.reporter = super.reporter;
     test('Compiler');
   }
 
   @override
-  Backend createBackend() {
+  JavaScriptBackend createBackend() {
     return new TestBackend(this);
   }
 
   @override
   ScannerTask createScannerTask() => new TestScanner(this);
 
   @override
   Resolution createResolution() => new TestResolution(this);
 
   @override
@@ skipping 243 matching lines @@
 
       expected =
           _expectedExitCode(beforeRun: tests[marker], fatalWarnings: true);
       totalExpectedErrors += expected.length;
       await testExitCodes(marker, expected, ['--fatal-warnings']);
     }
 
     Expect.equals(totalExpectedErrors, checkedResults);
   });
 }