How to mask a text with using a text layer

I would suggest a different workflow. Instead of using the text animator to animate the dots, create the dots as text on a path and then animate the path moving down. Then you can use that same path (linked with expressions) as a mask for a solid (or a path for a shape layer) that is used as a track matte for your text. (Basically what Rick said.)

Alternatively, if you're only doing one of these, just animate a mask on your letters to match the movement of the dots. It wouldn't take too long.

However, if you're dead set on doing it this way, duplicate your dots layer, use baseline shift, tracking, and font size as needed, and use it as a track matte. Might consider switching to a font that uses boxes like Redacted Font  or Blokk Font. Might use some blurring and then crushing of alpha with Levels to get it to be a bit smoother.

If I understand you correctly you want to have the animated dotted line reveal the text layer. I'm assuming you created the dotted line with a shape layer and animated the path. If that is the case then I would duplicated the shape layer, put one copy above the text layer, the other below, select the top shape layer, press the 'u' key twice to reveal all modified properties, turn off the stroke, add any solid color fill, and fix the animation so that you have a filled rectangle. 

You can then use the filled rectangle as an Inverted Alpha track matte for the text layer. The comp would look something like this:

You can see the path in the screenshot.