Prepare to publish stable null safety (dart-archive#178)

Bump SDK constraints for pub (dart-archive#37) (dart-archive#164)

Use a 2.12.0 lower bound since pub does not understand allowed
experiments for earlier versions.

Use a 3.0.0 upper bound to avoid a warning in pub and to give some
flexibility in publishing for stable.

remove git overrdies (dart-archive#163)

Prepare for the 2.11 dev SDKs (dart-archive#154)

Bump the upper bound to allow 2.10 stable and 2.11.0 dev SDK versions.

depend on 2.10 branch for sdk packages (dart-archive#150)

prepare for the 2.9 stable sdk release of package:collection (dart-ar…

…chive#148)

Updates the sdk constraint to <=2.9.10

rename version to -nullsafety (dart-archive#146)

Merge the null_safety branch into master (dart-archive#143)

Migrates the package to null safety

Prepare to publish (dart-archive#141)

CombinedMapView.keys fix (dart-archive#110)

Fixes https://github.com/dart-lang/collection/issues/109

Adds a custom iterable/iterator that can filter out duplicates and use that for the `CombineMapView.keys` getter. 

Updates tests to contain duplicates in maps, and ensure the keys/values from the earlier maps are the ones that are returned.

Updates the changelog and docs to no longer claim `O(maps)` for the length getter. This now requires iteration of all items and is `O(total map entries)`.

Prepare to publish as 1.14.12