Factor out some common logic from the launchers.

test/runner/browser/content_shell_test.dart

Index: test/runner/browser/content_shell_test.dart
diff --git a/test/runner/browser/content_shell_test.dart b/test/runner/browser/content_shell_test.dart
index 67100ec1ec476214005bcf0db783ec077df2dd31..e466a753770195169ef4737acd04b01b7a8143b1 100644
--- a/test/runner/browser/content_shell_test.dart
+++ b/test/runner/browser/content_shell_test.dart
@@ -150,6 +150,6 @@ webSocket.send(window.localStorage["data"].toString());
     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"))));
   });
 }