Migrate the example app to material_ui

[qeepcologne]

Material and Cupertino were decoupled from the Flutter SDK in 3.47 into the standalone material_ui / cupertino_ui packages. The in-SDK copies still work — they are frozen to contributions and scheduled for removal, with no date — so this is not urgent, but the example is the part of the repo people copy from, and it is the cheapest thing to move.

What changed

• 13 example sources + example/test/widget_test.dart: package:flutter/material.dart → package:material_ui/material_ui.dart, applied with dart fix --apply --code=migrate_design_widgets (it handles blog_tab.dart's hide Page correctly).
• example/pubspec.yaml: added material_ui: ^1.0.0, and raised environment from sdk: ">=3.0.3 <4.0.0" to sdk: ^3.12.0 + flutter: ">=3.44.0" — what material_ui 1.0.1 itself declares.
• Dropped two stale comments from the package pubspec.yaml (details below).

The library is untouched

git diff --stat -- lib/ is empty. The package keeps flutter: ">=1.17.0" and every dependency constraint is unchanged, so consumers are unaffected — only someone running the example needs Flutter 3.44+.

Known cosmetic effect

Once the example's MaterialApp is material_ui's, dependencies still on legacy Material resolve Theme.of against a theme that is no longer there. Of the example's UI dependencies only flutter_widget_from_html_core reads it (one call site), so HTML body text on the blog tab falls back to stock Material defaults. None of them call debugCheckHasMaterial*, so there is no assertion failure. Pulling in MaterialUiCompatibilityBridge to paper over one text style seemed worse than the fallback for a demo app.

Stale comments removed

Both were describing conditions that no longer hold:

• on intl — "flutter_localizations (Flutter SDK) pins intl to exactly 0.20.2". It declares ^0.20.3, and has since Roll pub packages (#188764) on 2026-06-29; the exact pin was dropped for good on 2026-04-21. The constraint itself is unchanged.
• on the absent analyzer pin — "breaks resolution on Flutter SDKs whose flutter_test pins the older meta 1.17.0". Current flutter_test pins meta: ^1.18.3, so that clash is gone.

Verification

• flutter analyze on the example: No issues found.
• flutter pub get resolves clean for both the package and the example.
• flutter test in the example still fails on Counter increments smoke test — but identically on unmodified main, which I confirmed with a throwaway worktree at HEAD. It is the stock template test pumping the real app, which needs .env Shopify credentials: ShopifyStore.getNProducts (shopify_store.dart:215:52) Null check operator used on a null value. Untouched here.

CI has to move too

The pinned flutter-version: '3.38.2' in .github/workflows/analyzer.yml ships Dart 3.10.0, and material_ui requires Dart ^3.12.0 / Flutter >=3.44.0 — so flutter pub get fails the job outright with "Because example requires SDK version ^3.12.0, version solving failed". There is no way to keep the migration and the old pin; I bumped it to 3.47.1 in a separate commit, which is the version I verified dart analyze --fatal-infos green on (clean checkout of this branch, fresh pub get, no local overrides). 3.44.0 is the true floor if you would rather move the minimum — I did not test that one, so I pinned what I actually ran.

If bumping your supported Flutter is not something you want to do right now, this PR is not worth it yet and I am happy to close it — the in-SDK Material still works and has no removal date.

Happy to split the comment removal out, or to drop the version bump if you would rather batch this into a later release.