Avoid race condition in async sample code

[jez]

r? @michelle

Summary & motivation

Fixes #154.

There are three solutions we can go with here:

1. check in constructor
2. check in componentDidMount
3. check in componentDidMount with a setTimeout

We've chosen to do the second, because

• It avoids the original race.
• It avoids recommending setTimeout as the default.
• It runs in componentDidMount, which is where browser-dependent code should
  live.

The drawbacks are

• It violates the "no setState in componentDidMount" idiom of React.
• It delays the time to first paint.

The "no setState" rule is fine; people who care will have ESLint complaint
that they're doing something weird. They can then make the decision to use this
solution or another.

For the others, it avoids the race with the simplest code.

[michelle]

lgtm!