public static WeakLineTracker Register(TextDocument textDocument, ILineTracker targetTracker)
{
if (textDocument == null)
throw new ArgumentNullException("textDocument");
if (targetTracker == null)
throw new ArgumentNullException("targetTracker");
var wlt = new WeakLineTracker(textDocument, targetTracker);
textDocument.LineTrackers.Add(wlt);
return wlt;
}