[pointer_interceptor] Migrate to null safety

[mzdm]

Related tracking issue flutter/flutter#75567

also:

• Added package links to package list to main README.md
• Added --web-renderer=html to integration test command
  (seems to be failing without that)
• Not sure what to return in case of Shim methods
  (methods from engine are non-nullable so I replaced null with an empty string)

[stuartmorgan-g]

@ditman should do the full review here.

[ditman]

Thanks for the ping! This is my next task, I'll take a look within ~24h :)

[ditman]

This looks great to me. I left some minor comments on the PR, and reversioned to 0.9.0. I've also migrated the integration_test code to null-safety!

Let's wait until #289 lands to merge this!

[ditman]

(The publishable check will fail until #289 lands, and this branch is rebased)

[mzdm]

(The publishable check will fail until #289 lands, and this branch is rebased)

Alright, thank you for checking this and making the other changes!

[stuartmorgan-g]

289 has landed, so you can merge in master and the test will pass.

[ditman]

Merged and published https://pub.dev/packages/pointer_interceptor/versions/0.9.0

It's still being listed as "Preview", I guess because of the version of the SDK we used being a "-0". We might have to revisit this to move 0.9.0 to stable.

[stuartmorgan-g]

Preview is different than Pre-release; it just means that Pub has noticed that it depends on a version of the Dart SDK that's not yet released to stable (in this case, 2.12). It should automatically change to being listed as stable once that happens (per the very end of this recent post).

[ditman]

Ooooh, cool, I missed that memo! I thought the publishing warning of "you're publishing a not-preview version that depends on a preview version of the SDK" had toggled something when I published. Thanks for the link!