Preserve ordering of constants rather than sorting them alphabetically

[jcollins-g]

Transfer from old dartdoc discussion:

"Frequently, specific parts of a class, file or library are part of a mental group. For example, in DateTime we can group JANUARY, FEBRUARY, etc. Furthermore, for some of them (like the months and weekdays) we want to keep them in a specific order."

[kevmoo]

This would be something to flag as an option.

In most cases, I'd argue for alphabetical sort – to ease scanning.

If a package author wants to opt-out, it should be explicit – perhaps via annotation?

[jcollins-g]

Agreed. Categorization such as that proposed in #1353 might help with this too, to group related items together in the docs.