Update code comments for navigation_bar.dart (#162596)
Fixes https://github.com/flutter/flutter/issues/162592
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### Comments error:
```
class NavigationDestination extends StatelessWidget {
...
/// The text label that appears below the icon of this
/// [NavigationDestination].
///
/// The accompanying [Text] widget will use
/// [NavigationBarThemeData.labelTextStyle]. If this are null, the default
/// text style would use [TextTheme.labelSmall] with [ColorScheme.onSurface].
final String label;
...
}
```
`If this are null, the default text style would use
[TextTheme.labelSmall]` is wrong. Actually, This is right in material2,
but wrong in material3. I have dived into `_NavigationBarDefaultsM3`, I
found this:
```
class _NavigationBarDefaultsM3 extends NavigationBarThemeData {
...
@override
MaterialStateProperty<TextStyle?>? get labelTextStyle {
return MaterialStateProperty.resolveWith((Set<MaterialState> states) {
final TextStyle style = _textTheme.labelMedium!;
return style.apply(
color: states.contains(MaterialState.disabled)
? _colors.onSurfaceVariant.withOpacity(0.38)
: states.contains(MaterialState.selected)
? _colors.onSurface
: _colors.onSurfaceVariant
);
});
}
...
}
```
**The default text style would use [TextTheme.labelMedium] in
Material3.**
### Changes
Just clarify the doc comments above.[test-exempt]
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---------
Co-authored-by: Taha Tesser <tessertaha@gmail.com>
f219bbac55