After some poking around, I found this button on the left side of the Build window:
Clicking this button toggles the view between the new “Build” view and a text output version which resembles the old Messages view – although it isn’t exactly the same.
I hope this helps!
UPDATE: In Android Studio 3.3 the button now looks like this:
UPDATE 2: In Android Studio 3.6 the button is gone. Instead, the build window is permanently split between text output and visual output. If you can’t find the text output, it might be fully collapsed, so look on the top right of the build window for something like this:
And try to drag it left to reveal the build window, like this:
Hope this helps!