Included source code no longer highlights

[jcollins-g]

Introduced by #1489, source code included in individual identifier pages is no longer highlighted.

example: https://api.dartlang.org/be/151273/dart-math/Point/magnitude.html

Filing as P1 (minor display warts not significantly impeding navigation, widespread).

[jcollins-g]

@kevmoo I'll take care of this eventually but tagging in case you're interested in fixing. Fixing the source code template html should do the trick.