…nt (#735)

* Update README.md

* address strict_top_level_inference diagnostics

* deprecate UnmodifiableMapMixin.first and UnmodifiableMapMixin.last

* Update unmodifiable_wrappers.dart

* ignore provide_deprecation_message diagnostics

Closes #845

It is idiomatic to treat the key order of a Dart map as meaningful
given that map literals and default Map type preserve key insertion
order. It is more useful to allow the caller to decide this order than
to mandate an alpha sorting by key. Callers which need this order can
construct the map appropriately, and callers which prefer a different
order now have the capability.

Remove the additional list copy and iterate the map keys directly.

Releasing as a non-breaking change since specific usage output is
considered an implementation detail. This is expected to impact some CI
statuf for packages with tests hardcoding a strict dependency on the
output.

No additional tests are necessary since updating the order in existing
tests demonstrates the same behavior as adding a non-sorting specific
test.

Refactor a few null check conditions to variable assignment patterns.