Factor out some common logic from the launchers.

test/runner/browser/content_shell_test.dart

 // Copyright (c) 2015, 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.
 
 @TestOn("vm")
 
 import 'dart:async';
 
 import 'package:shelf/shelf.dart' as shelf;
 import 'package:shelf/shelf_io.dart' as shelf_io;
@@ skipping 132 matching lines @@
       await contentShell.close();
     } finally {
       server.close();
     }
   });
 
   test("reports an error in onExit", () {
     var contentShell = new ContentShell("http://dart-lang.org",
         executable: "_does_not_exist");
     expect(contentShell.onExit, throwsA(isApplicationException(startsWith(
-        "Failed to start content shell: $noSuchFileMessage"))));
+        "Failed to run Content Shell: $noSuchFileMessage"))));
   });
 }