--format=machine parameter for flutter analyze

[twogood]

@christopherfujino

Use case

dart analyze has a parameter --format:

    --format=<value>         Specifies the format to display errors.

          [default]          The default output format. This format is intended to be user consumable.
                             The format is not specified and can change between releases.
          [json]             A machine readable output in a JSON format.
          [machine]          A machine readable output. The format is:

                             SEVERITY|TYPE|ERROR_CODE|FILE_PATH|LINE|COLUMN|LENGTH|ERROR_MESSAGE

                             Note that the pipe character is escaped with backslashes for the file path and error message fields.

The output from dart analyze --format=machine is supported by https://github.com/tomasbjerre/violation-comments-action so that it is possible to get analysis comments directly in PR diffs like this but with Dart code:

It might therefore be considered advantageous if flutter analyze also had this feature for consistency. Note: There may be other dart analyze parameters that are not available for flutter analyze but those are not part of this feature request.

Proposal

I propose the implementation of flutter analyze --format=…. The means would mean changing at least:

• https://github.com/flutter/flutter/blob/master/packages/packages/flutter_tools/lib/src/commands/analyze.dart to add argParser.addOption('format', …
• https://github.com/flutter/flutter/blob/master/packages/packages/flutter_tools/lib/src/commands/analyze_base.dart to pick up argResults['format'] and write the specified output format

We need the same or almost the same code as emitMachineFormat and emitJsonFormat from https://github.com/dart-lang/sdk/blob/main/pkg/dartdev/lib/src/commands/analyze.dart and I suggest we just copy that code and modify as needed. In the name of reuse, it might be considered to introduce a pub.dev package but I personally believe that copying code is a better option in this case.