null-safety incompatible with beta and dev releases

[muhlba91]

Flutter bumped 1.26 to beta and releases 1.27 on dev.
They also bumped some dependencies to null-safety versions by default and, since get_it requires certain tagged pre-release versions of those, the current null-safety prerelease is not compatible and working anymore:

Because no versions of get_it match >6.0.0-nullsafety.2 <7.0.0 and get_it 6.0.0-nullsafety.2 depends on collection 1.15.0-nullsafety.5, get_it ^6.0.0-nullsafety.2 requires collection 1.15.0-nullsafety.5.
And because every version of flutter from sdk depends on collection 1.15.0, get_it ^6.0.0-nullsafety.2 is incompatible with flutter from sdk

In particular, updates affect:

• collection to 1.15.0
• meta to 1.3.0
• async to 2.5.0