How does Google measure its programmers productivity?

How does Google measure its programmers productivity?

In an article (Google's "20% time," which brought you Gmail and AdSense,
is now as good as dead) on Quartz, the author writes, "Managers are judged
on the productivity of their teams—Google has a highly developed internal
analytics team that constantly measures all employees' productivity..."
My question is this: How does Google measure the productivity of its
programmers?
I found a couple of questions about measuring productivity, but they were
more vague than my question (e.g. "how can I measure productivity"). I
think my question has value because Google is known to place a high value
on meaningful analytics and because they are well respected within the
programming community. Rather than being a high-level open-ended question,
there is a definite answer to this question.