Crash when building Angular docs

[kwalrath]

For details, see https://github.com/dart-lang/site-webdev/issues/296.

[devoncarew]

@astashov, see the exception near the end of https://s3.amazonaws.com/archive.travis-ci.org/jobs/188681711/log.txt

I think this was caused by #1313; do you mind taking a look?

[astashov]

Sure!

[kwalrath]

Thanks for the quick fix!