* First step in Flutter Doctor refactor. Assigns categories to all validators.
* Revert "Roll engine e54bc4ea1832..a84b210b3d26 (6 commits) (#20453)"
This reverts commit 05c2880a1736bbc90c8870a3b498fab074b0d4c0.
* 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.
Seems like VS Code may have started using lowercase extension folders recently (our identifier is `Dart-Code.flutter` but now the folder is named `dart-code.flutter`), so this makes the check not sensitive to casing.
Also reuse extension identifier for download link
Looks in default install locations on Mac, Linux and Windows for VS Code. If found, looks in default extension location to see if Dart Code is installed.
If VS Code is not installed, nothing is reported. If VS Code is installed without Dart Code, a warning is shown.
