DownloadStringCompletedEventArgs no longer valid?

This function and all like it no longer work:

private void CompleteCheckLoginStatus(object sender, DownloadStringCompletedEventArgs e)

The error I get is: The type or namespace name 'DownloadStringCompletedEventArgs' could not be found

 I didn't see anything in the breaking changes regarding this, did something change with beta 2?
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