SkinMinerva class has some logic related to the Echo extension (useEcho(), getEchoNotifUser(), getEchoSeenTime(), getFormattedEchoNotificationCount()). Instead of hardcoding Echo-related logic inside Skin class, please move the echo code into the Echo extension.
This code would ideally live in Echo, and be registered via hook. By default where Echo is not installed we show a link to the user's talk page. Echo would be able to replace that button with the Notifications link and register the code.