No code changes, just comments: I spell-checked all the comments in the repo.

This reverts commit 421bf647.

Was causing severe breakages in Google.

Update the Slider and RangeSlider to the latest Material spec. This introduces an updated track, updated thumbs, updated tick marks, and a new value indicator shape. The old paddle value indicator shape is also updated so that text scaling works consistently.

* Updates to slider_test.dart

Fixed error that required onChanged.

* Updated slider.dart and slider_test.dart debugFillProperties

Updated slider.dart debugFillProperties to include all properties and slider_test debugFillProperties to test all properties

* Updated Slider onChanged property

* Updated debugFillProperties to ignore testing methods that are null in test

Per feedback from Hans/Anthony on Flutter Material

* Updated spacing in debugFillProperties

Per feedback from Hans

* Updated range_slider.dart and range_slider_test.dart to have DiagnosticProperties tests for all properties

* Updated range_slider.dart and range_slider_test.dart per feedback

Preventing null labels from creating an error.

Added made RangeLabels and RangeValues constants in instantiation of Range Slider.

Fixed spacing.

* Updates to start and end labels in diagnostic property tests

Updated per feedback from @clocksmith

* Implement system fonts system channel listener

(Breaking Change) Move some hardcoded pixel values to reusable constants.

[Material] Text scale and wide label fixes for Slider and Range Slider value indicator shape (#35496)

Various bug fixes for Slider and Range Slider value indicator around accessibility and wide labels.

Fixes a bug in the Range Sider where the onChangeStart and onChangeEnd were not being called if the initial thumb selection came from a drag (rather than a tap).

[Material] Fix TextDirection and selected thumb for RangeSliderThumbShape and RangeSliderValueIndicatorShape (#34501)

Expose the RangeSlider's TextDirection in the RangeSliderThumbShape's abstract paint() method, and ensure that start and end thumb are non-null when being passed to the thumb and value indicator shape paint methods.

Creates a Material Design range slider.

The range slider is based off the updated slider, with the main difference being that it has 2 thumbs.
By default, the thumbs cannot pass each other and both the thumb and the value indicator create a stroke when they are overlapping for better visibility.