Support default document (index.html) for cleaner URLs

[eseidelGoogle]

Right now to actually get to a library root page you need to know dartdocs pattern of doing name-library.html:
https://docs.flutter.io/flutter/dart-async/dart-async-library.html

https://docs.flutter.io/flutter/dart-async/
would just get you a 404.

Obviously we could fix this in Flutter's doc generation script as well, but seems like something maybe all dartdoc users might want?

[sethladd]

Presumably this means there is an index.html under dart-async/. Could there be a thing called "index" inside of a Dart library? If so, what kind of structure would we need to put all the things into sub-directories so that they don't collide with a name called "index" ?

We might be able to teach Firebase what the "index.html" file is actually called. But, that would be a solution specific to Flutter's docs. I think assuming an "index.html" is a safe assumption, so many web servers will look for that file.

Just need to be careful here with collisions... but yeah, +1 for anything we can do for more beautiful URLs

[kevmoo]

https://api.dartlang.org/stable/1.24.3/dart-core/

Is soooo much nicer than

https://api.dartlang.org/stable/1.24.3/dart-core/dart-core-library.html

[kevmoo]

CC @athomas