Migrate embedded DartPad editors away from multi-file

[johnpryan]

Describe the problem

We need to remove any content that uses DartPad's editable test code features so that they are ready to migrate to the new preview.dartpad.dev editor.

Expected fix

Pages that use embedded DartPads:

• Implicit animations codelab
• Flutter Cookbook
• Write your first Flutter app on the web
• Building user interfaces with Flutter
• Using Actions and Shortcuts
• Understanding constraints

Pages that mention DartPad workshops:

• Codelabs & workshops
• Using slivers to achieve fancy scrolling

On which platform(s) did you experience this issue?

No response

Additional context

See also: https://github.com/dart-lang/site-www/issues/5382

[atsansone]

@johnpryan : Could use some clarity for this issue. Do you want the embedded DartPad removed on these pages? If it appears on other pages, should we remove them there, too? With the pages that mention DartPad, we should remove the mention?

Thanks!

[parlough]

I think I've removed the remaining use cases of multi-file support already (on docs.flutter.dev), but if someone wants to double check, please do so!

The mentions to workshops will have to be removed though.

[johnpryan]

Tony, I shared the migration plan with you at go/dartpad-frontend-migration-plan.

I haven't verified that the test code features have been removed, so I filed this issue to be safe, but if none of them are using these features we should be fine to close this issue.

[atsansone]

That works. Thanks @johnpryan !

[parlough]

I'll do another pass to make sure and handle any other remaining uses and workshop mentions. Thanks for filing the issue John!