[material_ui, cupertino_ui] Add migration bridge utilities

[Piinks]

This adds utilities to the material_ui and cupertino_ui to help cover the gap during the ecosystem migration. Users who have migrated their applications can use these utilities to retain compatibility with packages that have not migrated yet.
These are marked deprecated as they will be removed in a future major release.

Example:

import 'package:material_ui/material_ui.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        colorScheme: ColorScheme.fromSeed(seedColor: const Color(0xFF6750A4)),
      ),
      builder: (BuildContext context, Widget? child) {
        return MaterialUiCompatibilityBridge(child: child!);
      },
      home: const HomeScreen(),
    );
  }
}

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

Note: The Flutter team is currently trialing the use of Gemini Code Assist for GitHub. Comments from the gemini-code-assist bot should not be taken as authoritative feedback from the Flutter team. If you find its comments useful you can update your code accordingly, but if you are unsure or disagree with the feedback, please feel free to wait for a Flutter team member's review for guidance on which automated comments should be addressed.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩²

[gemini-code-assist]

Code Review

This pull request introduces CupertinoUiCompatibilityBridge and MaterialUiCompatibilityBridge to facilitate migration by mapping modern theme and localization configurations to their legacy equivalents. Feedback on the changes suggests mapping the platform and visualDensity properties in MaterialUiCompatibilityBridge to ensure consistent platform-specific behaviors and layout densities.

[gemini-code-assist]

The platform and visualDensity properties are not mapped from modernTheme to the legacy ThemeData. This can cause inconsistent platform-specific behaviors (e.g., scroll physics, adaptive widgets) and layout densities for legacy widgets inside the bridge. Consider mapping these properties as well.

Suggested change

	return legacy.ThemeData(
	colorScheme: legacy.ColorScheme(
	return legacy.ThemeData(
	platform: modernTheme.platform,
	visualDensity: modernTheme.visualDensity,
	colorScheme: legacy.ColorScheme(

[justinmc]

Good catch Gemini, or did you purposely omit these @Piinks?

[Piinks]

Definitely a good catch!

[justinmc]

LGTM 👍

[justinmc]

Let's keep an eye out for any side effects of this import, but I can't think of any specific reason why it should complicate things.

[justinmc]

Nit: I would compare with modernTextTheme.navTitleTextStyle.fontSize instead of hard coding 20.0, and so on. That way it's clear to a reader where that value came from, and there's less chance of things getting out of sync.

[justinmc]

Good catch Gemini, or did you purposely omit these @Piinks?

[Piinks]

Feedback addressed, PTAL! 🙂

[justinmc]

LGTM 👍