Added tool to easily check golden diffs locally.

[gaaclarke]

This addresses the problem where people feel like they have to upload a PR to get diffs from Skia Gold. This should work for most workflows.

Pre-launch Checklist

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact "@test-exemption-reviewer" in the #hackers channel in Chat (don't just cc them here, they won't see it! Use Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

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[gaaclarke]

This could have been an external tool, it's so fundamental and tiny though I wanted to integrate it with the engine repo.

[matanlurey]

I'd like to see this in tools/*, and follow the same structure as tools in that directory (including tests).

That being said, I also need that tool to exist (flutter/flutter#143459) - so I'm happy to have you merge this and then I can take that part over, up to you.

[gaaclarke]

Yea, this happened because I started writing it in bash, then realized writing it in Dart would be better.

That being said, I also need that tool to exist (flutter/flutter#143459) - so I'm happy to have you merge this and then I can take that part over, up to you.

What did you mean by this? What's this have to do with that other proposed tool? Just that you'd be in there anyways? I'm happy to make it a more fleshed out tool. This was so tiny I was hoping this would be all that's required.

[matanlurey]

Oh sorry I misunderstood what this is/isn't. Sgtm, I think having this is strictly better than not.

I do think we should put it in tools/ just to be consistent and not start accumulating Dart scripts all over.

[gaaclarke]

I do think we should put it in tools/ just to be consistent and not start accumulating Dart scripts all over.

Done. I added all the stuff to write tests. I didn't add one, I can if we want. It requires some noodling with directories and mocking which adds extra complexity to the script.