Dart glossary is nearly empty

[kwalrath]

The Dart glossary used to be mostly the same as the TS glossary.

Now it contains only a definition for snake_case.

It should look more like this archived copy: https://web.archive.org/web/20160304140150/https://angular.io/docs/dart/latest/guide/glossary.html

[kwalrath]

cc @chalin