[cupertino_ui] fix material_ui cross imports in cupertino_ui samples

[navaronbracke]

This PR fixes cross imports of Material / material_ui in the cupertino_ui API examples.

Doc imports for material_ui are left as is, since I'm not sure what to do with those yet.

This was originally flagged in flutter/flutter#187662
because the new checker flagged 4 of the examples having cross imports.

Note: the generic text_magnifier sample was not flagged by the PR above, but it did have a cross import.
This is because the example was in the wrong place as noted by a1682d4

See https://github.com/flutter/flutter/pull/187662/changes#diff-44a841a9d512f4332f43cceddafbe29069dfaea081ae0adad3a9990c13816006R123-R126

Part of flutter/flutter#187645

Since the Code Freeze period is still ongoing, I did not chance the version for cupertino_ui in the changelog yet.

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[navaronbracke]

Since the examples were relying on the Material Icons, I opted to switch to CupertinoIcons to fix those cross imports.

[gemini-code-assist]

Code Review

This pull request removes material_ui imports and dependencies from the cupertino_ui package examples and tests, replacing Material widgets, icons, and colors with Cupertino equivalents. The reviewer suggests removing the unused material_ui dependency from pubspec.yaml to clean up the configuration.

[gemini-code-assist]

Since all cross-imports of material_ui have been removed from the cupertino_ui examples and tests, the material_ui dependency (and its path override) in pubspec.yaml is no longer needed and can be safely removed to keep the dependencies clean.

[navaronbracke]

This is not true, since there are @docImports of material_ui, which this PR does not fix.

[navaronbracke]

This one was not disposed :(

[navaronbracke]

For CupertinoTextField, instead of double tapping the word, I had to use long press to make it show up.

[Piinks]

I am going to move this to a draft for now since we are not ready to start taking contributions that we have not identified as critical to get to our initial release. We also do not want other contributors to mistake these packages as ready for PRs yet and flood the queue while we are focusing on the most essential work to be able to ship. This is also a more significant refactor I want to make sure we review thoroughly. :)
FYI @justinmc in case we determine we need to do this to remove the dev dependency, which might be an issue we ran into today.

[navaronbracke]

Agreed! Yeah I just put this up so I wouldn't lose track of it.

[Piinks]

in case we determine we need to do this to remove the dev dependency, which might be an issue we ran into today.

It might actually be ok. These are just the examples, which IIRC, do not require the package itself to take on the dependency.

In general, I think it is ok to have cross imports in the examples of the package? Examples can use all sorts of other packages for demonstration, why should they be removed?