* First step in Flutter Doctor refactor. Assigns categories to all validators.

* Revert "Roll engine e54bc4ea1832..a84b210b3d26 (6 commits) (#20453)"

This reverts commit 05c2880a.

* Split iOS and Android workflows into workflow and validator classes.

* Change ValidatorCategory to handle standalone validators that share a
category (e.g. IntelliJ).

Also make Android Studio and Android toolchain use separate categories.

At this stage, flutter doctor output matches what it was previously.
(The summary() method itself has not yet been changed )

* Change doctor summary code to support validator categories.

Output is still unchanged.

* Handle small formatting issues.

* Flip Flutter category's isGroup field to false until it's actually
needed.

* Revert auto-generated formatting changes to keep those lines from
muddying the pull.

* Small fixes pointed out by analyzer.

* Properly fix analyzer issues around const constructors.

* Small changes to address comments.

* Add tests to verify grouped validator behavior and validationtype
merging.

* Update doctor.dart

* Add comments for clarification.