[url_launcher] migrate url_launcher package to null safety

[mvanbeusekom]

Description

This pull request add null safety support to the url_launcher package.

The implementation depends on companion PR #3142 which add null safety support to the url_launcher_platform_interface package. Since PR #3142 isn't merged as of yet the dependency is currently based on a path reference, which should be fixed before merging this PR.

Related Issues

flutter/flutter#66395

Checklist

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• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
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Breaking Change

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[blasten]

Let me know when this is ready to review

[mvanbeusekom]

@blasten, it would be great if you can do a review on the code. I think everything looks good, I am just waiting on a reply from David regarding some information why the integration tests won't run (I think this is related to the changes in #3158 which adds nnbd support in the integration_test package).

[blasten]

@mvanbeusekom would you mind rebasing your branch?

[blasten]

why not 6.0.0?

[mvanbeusekom]

I updated the version number in the middle of the discussion we had for the url_launcher_platform_interface. Since for that plugin we settled on doing a major release I think it should be good to do it here as well.

So I updated the version to 6.0.0-nullsafety. Main argument the canLaunch and launch methods won't be returning an optional null value anymore (where in earlier versions this was a possibility).

[blasten]

end of file

[mvanbeusekom]

Fixed.

[blasten]

do we need to do web as well? cc @ditman

[ditman]

I'm thankful this is migrated, however integration_tests aren't currently running.

[ditman]

circling back, you guys discussed offline to revert url_launcher_web. I'm fine with that. Thanks for the effort @mvanbeusekom!

[mvanbeusekom]

@ditman Maybe I can revert it back in this PR and open a second one just for the url_launcher_web package as draft and have it ready to go when all dependencies get fixed?

[ditman]

@mvanbeusekom that sounds great, that way all the effort doesn't go to waste! :)

[mvanbeusekom]

@mvanbeusekom would you mind rebasing your branch?

Thanks for the review, I rebased the branch and also processed your other feedback

[blasten]

LGTM