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1. Ports `FlutterTask` from groovy to kotlin 2. Refactors `BaseFlutterTask` a bit, to follow a paradigm that methods on `BaseFlutterTask` and `FlutterTask` will contain no logic, and instead be set equal to a method with the same name in `BaseFlutterTaskHelper` and `FlutterTaskHelper` respectively, and that both of those helpers will be stateless. This allows us to somewhat get around the fact that you cannot create a `Task` instance in testing, because testing the logic of the task is the same as testing the logic of the helper. 3. Also creates a `FlutterPluginConstants` object to hold the constants from `FlutterPlugin`. I think ideally this will be collapsed back in to `FlutterPlugin` when the conversion is completed, but it needs to be moved for now so it can be referenced in Kotlin code (this pr needs it). Fixes https://github.com/flutter/flutter/issues/162112 ## Pre-launch Checklist - [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs. - [x] I read the [Tree Hygiene] wiki page, which explains my responsibilities. - [x] I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement]. - [x] I signed the [CLA]. - [x] I listed at least one issue that this PR fixes in the description above. - [x] I updated/added relevant documentation (doc comments with `///`). - [x] I added new tests to check the change I am making, or this PR is [test-exempt]. - [x] I followed the [breaking change policy] and added [Data Driven Fixes] where supported. - [x] All existing and new tests are passing. If you need help, consider asking for advice on the #hackers-new channel on [Discord]. <!-- Links --> [Contributor Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview [Tree Hygiene]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md [test-exempt]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests [Flutter Style Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md [Features we expect every widget to implement]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement [CLA]: https://cla.developers.google.com/ [flutter/tests]: https://github.com/flutter/tests [breaking change policy]: https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes [Discord]: https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md [Data Driven Fixes]: https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md --------- Co-authored-by: Gray Mackall <mackall@google.com>