Reduce repetitive Theme.of() calls

[jpnurmi]

It's not an expensive call, but not free either. Every Theme.of() call implies a Localizations.of() call, so two inherited widget lookups both of which are basically hashmap lookups. Last but not least, the two are merged and cached, resulting in a third hashmap lookup. Therefore it's nice to store the theme data to a variable at the beginning of build() to avoid repeating the hashmap lookups several times.

  static ThemeData of(BuildContext context) {
    final _InheritedTheme? inheritedTheme = context.dependOnInheritedWidgetOfExactType<_InheritedTheme>(); // <== 1
    final MaterialLocalizations? localizations = Localizations.of<MaterialLocalizations>(context, MaterialLocalizations); // <== 2
    final ScriptCategory category = localizations?.scriptCategory ?? ScriptCategory.englishLike;
    final ThemeData theme = inheritedTheme?.theme.data ?? _kFallbackTheme;
    return ThemeData.localize(theme, theme.typography.geometryThemeFor(category)); // <== 3
  }

https://github.com/flutter/flutter/blob/5c97543027f62bf73062db5e06b1b1b9d5636dd1/packages/flutter/lib/src/material/theme.dart#L107-L113