[kevmoo]

Avoid looking up InheritedCupertinoTheme on every Theme.of(context) call when a Material _InheritedTheme ancestor is already present, and break the Theme.build -> MaterialBasedCupertinoThemeData -> Theme.of -> CupertinoBasedMaterialThemeData -> ColorScheme.fromSeed retention cycle so Dart TFA can tree-shake ColorScheme.fromSeed and package:material_color_utilities when unused.

• Problem:
  1. Theme.of(context) unconditionally called context.dependOnInheritedWidgetOfExactType<InheritedCupertinoTheme>() before checking inheritedTheme != null, registering a redundant InheritedElement dependency on every widget calling Theme.of(context) even when a Material Theme ancestor was present.
  2. Theme.build wraps every Material Theme in CupertinoTheme(data: MaterialBasedCupertinoThemeData(...)), and MaterialBasedCupertinoThemeData evaluated (materialTheme.cupertinoOverrideTheme ?? const CupertinoThemeData()).noDefault(), instantiating const CupertinoThemeData() in every Material app.
  3. Because InheritedCupertinoTheme and CupertinoThemeData were always instantiated, Dart Type Flow Analysis (TFA) could not eliminate the CupertinoBasedMaterialThemeData fallback branch in Theme.of(context), which calls ColorScheme.fromSeed and retains all 146 CAM16/HCT solver functions in package:material_color_utilities even in apps that never use ColorScheme.fromSeed.
• Change:
  • Only look up InheritedCupertinoTheme in Theme.of(context) when inheritedTheme == null.
  • Replace (materialTheme.cupertinoOverrideTheme ?? const CupertinoThemeData()).noDefault() with materialTheme.cupertinoOverrideTheme?.noDefault() ?? const NoDefaultCupertinoThemeData() in MaterialBasedCupertinoThemeData.
  • Return themeData._materialTheme from CupertinoBasedMaterialThemeData when themeData is a MaterialBasedCupertinoThemeData.
• Measured Wasm Size Impact (slide_puzzle with flutter build web --wasm --no-strip-wasm --source-maps):
  • Raw .wasm size: 2,497,700 B -> 2,453,498 B (-44,202 B / -1.77%; -35,403 B / -1.42% without --source-maps)
  • Wasm code section: 1,003,395 B -> 984,868 B (-18,527 B / -1.85%, -166 functions)
  • Theme.of compiled function size: 1,633 B -> 361 B (-1,272 B / -77.89%)
  • package:material_color_utilities retained code: 17,096 B (146 functions) -> 24 B (2 equality stubs, -99.86%)

Part of flutter/flutter#188444

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[gemini-code-assist]

Code Review

This pull request optimizes theme lookups and enables better tree-shaking. Specifically, it avoids looking up InheritedCupertinoTheme in Theme.of when a Material Theme ancestor is already present. It also updates MaterialBasedCupertinoThemeData to use NoDefaultCupertinoThemeData as a fallback, and modifies CupertinoBasedMaterialThemeData to preserve the underlying ThemeData when the provided theme is a MaterialBasedCupertinoThemeData. Corresponding unit tests and a changelog entry have been added. There are no review comments, and I have no feedback to provide.