docs(widget-previewer): update examples to match PreviewThemeData API

[maranix]

Fixes #13799

• Align Custom and Transformative Preview examples with recent PreviewThemeData API changes in docs.

• Added appropriate Screenshot and GIF for visual reference

Presubmit checklist

• If you are unwilling, or unable, to sign the CLA, even for a tiny, one-word PR, please file an issue instead of a PR.
• If this PR is not meant to land until a future stable release, mark it as draft with an explanation.
• This PR follows the Google Developer Documentation Style Guidelines—for example, it doesn't use i.e. or e.g., and it avoids I and we (first-person pronouns).
• This PR uses semantic line breaks
  of 80 characters or fewer.

[maranix]

Is gemini-code-assist not familiar with Primary Constructors and dot-shorthand?

Official Dart repo does provide skills for that so it should be, unless it is not using them. Anyways if there is a need for the examples to be in standard "old" dart then let me know and i will update them.

[gemini-code-assist]

Yes, I am familiar with those features. Using Primary Constructors and dot-shorthand is an excellent way to keep the documentation examples modern and concise, aligning with current Dart best practices. They are indeed the correct, idiomatic ways to represent these definitions in modern Dart.

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit ec9b098):

https://flutter-docs-prod--docs-pr13861-fix-13799-m9lmfd1o.web.app

[flutter-website-bot]

Staged preview of the updated flutter.dev site (updated for commit ec9b098):

https://flutter-dev-230821--www-pr13861-fix-13799-8ssrrdlm.web.app

[jesskuras]

Hi @maranix, thanks for sending this in to address #13799!

A few quick initial notes from triage:

Please review and check off the items in the Presubmit checklist in the PR description.
For the animation asset, we are currently migrating all GIFs across the site to WebP (see #13860)—could you provide this as a WebP instead of a GIF?

[maranix]

@jesskuras

• Updated Presubmit checklist
• Replaced transformative-preview.gif with transformative-preview.webp

Re-ran tests and built the docs website along with check-links and check-link-references.

Thanks and let me know if there is anything else is required.

[jesskuras]

Thanks for updating the presubmit checklist and converting the asset to WebP, @maranix!
(And yes, agreed, I think we can ignore those automated Gemini comments.)

@bkonyi @conooi — Could you take a look when you have a moment and see if you want to merge these changes in? Thanks so much! :)

[sfshaza2]

/gcbrun

[sfshaza2]

Is it just me, or is the bot sounding huffy when questioned?

[sfshaza2]

@parlough, isn't the website build using the latest version of Dart, namely 3.13?

[bkonyi]

LGTM!