Resolve linter warnings

[suragch]

There are a number of linter warnings. These should be resolved.

• 'hashValues' is deprecated and shouldn't be used. Use Object.hash() instead. This feature was deprecated in v3.1.0-0.0.pre.897.
  Try replacing the use of the deprecated member with the replacement.
• Avoid leading underscores for local identifiers.
• Sort child properties last in widget instance creations.
• Avoid leading underscores for local identifiers.
• A value for optional parameter 'child' isn't ever given.
• Avoid using private types in public APIs.
• 'accentColor' is deprecated and shouldn't be used. Use colorScheme.secondary instead. For more information, consult the migration guide at https://flutter.dev/docs/release/breaking-changes/theme-data-accent-properties#migration-guide. This feature was deprecated after v2.3.0-0.1.pre..
• 'fixTextFieldOutlineLabel' is deprecated and shouldn't be used. This "fix" is now enabled by default. This feature was deprecated after v2.5.0-1.0.pre..
• 'primaryColorBrightness' is deprecated and shouldn't be used. No longer used by the framework, please remove any reference to it. This feature was deprecated after v2.6.0-11.0.pre..
• 'vsync' is deprecated and shouldn't be used. This field is now ignored. This feature was deprecated after v2.2.0-10.1.pre..
• 'fadeDuration' is deprecated and shouldn't be used. Use SelectionOverlay.fadeDuration instead. This feature was deprecated after v2.12.0-4.1.pre..
• TODOs
• and more...