Broaden dependency version ranges

[Hixie]

Is there any reason we have to have the narrow ranges in pubspec.yaml? It's going to keep causing problems like we just had with intl, where it was preventing flutter from rolling to a newer version (e.g. right now we're pinned to <0.20, which means as soon as intl upgrades to 0.20, we'll be blocked again).

Is there any reason we can't just use any or just a really broad >0.18 range or some similar thing, and rely on testing in the client packages to catch problems?

[devoncarew]

FWIW, this package is published - https://pub.dev/packages/unified_analytics - but is unlisted (it doesn't show up in pub search results).

For published packages we do want to follow semver for dependency constraints. It adds a level of predictability for user's CI breakages and such.

If we don't expect usage outside of the dart sdk and flutter sdk (devtools? other published packages?) there may be ways to distribute other then publishing.

(context on the pubspec in question: https://github.com/dart-lang/tools/blob/main/pkgs/unified_analytics/pubspec.yaml#L17)