How do you focus a landscape shot when it is too dark to see anything through the viewfinder?

How to focus when it is too dark to see

Do the focusing in daylight. The accepted answer's method is to focus on something extremely distant while you can still see, mark that position on the focus ring, and return to the mark in the dark — because the infinity mark printed on a lens is often not exactly where infinity is.

Preparing in advance

In daylight, focus carefully on something very far away and mark the focus ring at that position with tape.
That mark is your infinity setting, and it can be found by touch when nothing is visible.
It survives being carried around, which is the point — the work is done before you are standing in the dark.

Giving the camera something to see

A laser pointer fixed to the camera or tripod puts a visible dot on the subject, bright enough to focus on and to check by eye.
A torch on the subject serves the same purpose at closer distances, then gets switched off before the exposure.
Both work by supplying contrast where there is none, which is what the autofocus system is missing rather than light as such.

When there is genuinely nothing visible

One answer drew a useful distinction between low light and no light. Several techniques assume you can see something faintly through the viewfinder or that the subject is close enough for a focusing aid to reach it. For a landscape at night neither may be true, and the methods that work are the ones that do not require seeing the subject at all — which is why the daylight mark is the most reliable of them.

Focusing on stars

Pick a bright star and magnify it, using either the camera's live view magnification or a magnifying angle finder.
A focusing mask placed over the lens produces a diffraction pattern that snaps into symmetry at exact focus, which gives a definite target instead of a judgement call.
Stars are the hardest case and the most objective: at correct focus a star becomes a point, and any error is immediately visible.