Dart-sass

* Don't emit permissions errors on Windows and OS X when trying to determine the
real case of path names.

* Deprecate passing non-`%` numbers as lightness and saturation to `hsl()`,
`hsla()`, `color.adjust()`, and `color.change()`. This matches the CSS
specification, which also requires `%` for all lightness and saturation
parameters. See [the Sass website][color-units] for more details.

* Deprecate passing numbers with units other than `deg` as the hue to `hsl()`,
`hsla()`, `adjust-hue()`, `color.adjust()`, and `color.change()`. Unitless
numbers *are* still allowed here, since they're allowed by CSS. See [the Sass
website][color-units] for more details.

* Improve error messages about incompatible units.

* Properly mark some warnings emitted by `sass:color` functions as deprecation
warnings.

Dart API

* Rename `SassNumber.valueInUnits()` to `SassNumber.coerceValue()`. The old name
remains, but is now deprecated.

* Rename `SassNumber.coerceValueToUnit()`, a shorthand for
`SassNumber.coerceValue()` that takes a single numerator unit.

* Add `SassNumber.coerceToMatch()` and `SassNumber.coerceValueToMatch()`, which
work like `SassNumber.coerce()` and `SassNumber.coerceValue()` but take a
`SassNumber` whose units should be matched rather than taking the units
explicitly. These generate better error messages than `SassNumber.coerce()`
and `SassNumber.coerceValue()`.

* Add `SassNumber.convertToMatch()` and `SassNumber.convertValueToMatch()`,
which work like `SassNumber.coerceToMatch()` and
`SassNumber.coerceValueToMatch()` except they throw exceptions when converting
unitless values to or from units.

* Add `SassNumber.compatibleWithUnit()`, which returns whether the number can be
coerced to a single numerator unit.

* Add support for parsing `clamp()` as a special math function, the same way
`calc()` is parsed.

* Properly load files in case-sensitive Windows directories with upper-case
names.

* Fix a bug where `at-root (without: all)` wouldn't properly remove a
`keyframes` context when parsing selectors.

Node JS API

* The generated `main()` function in `sass.js` now returns a `Promise` that
completes when the executable is finished running.

Dart API

* Fix a bug that prevented importers from returning null when loading from a
URL that they had already canonicalized.

* Support a broader syntax for `supports` conditions, based on the latest
[Editor's Draft of CSS Conditional Rules 3]. Almost all syntax will be allowed
(with interpolation) in the conditions' parentheses, as well as function
syntax such as `supports selector(...)`.

[Editor's Draft of CSS Conditional Rules 3]: https://drafts.csswg.org/css-conditional-3/#at-supports

* Add a [`color.hwb()`] function to `sass:color` that can express colors in [HWB] format.

[`color.hwb()`]: https://sass-lang.com/documentation/modules/color#hwb
[HWB]: https://en.wikipedia.org/wiki/HWB_color_model

* Add [`color.whiteness()`] and [`color.blackness()`] functions to `sass:color`
to get a color's [HWB] whiteness and blackness components.

[`color.whiteness()`]: https://sass-lang.com/documentation/modules/color#whiteness
[`color.blackness()`]: https://sass-lang.com/documentation/modules/color#blackness

* Add `$whiteness` and `$blackness` parameters to [`color.adjust()`],
[`color.change()`], and [`color.scale()`] to modify a color's [HWB] whiteness
and blackness components.

[`color.adjust()`]: https://sass-lang.com/documentation/modules/color#adjust
[`color.change()`]: https://sass-lang.com/documentation/modules/color#change
[`color.scale()`]: https://sass-lang.com/documentation/modules/color#scale

Dart API

* Add [HWB] support to the `SassColor` class, including a `SassColor.hwb()`
constructor, `whiteness` and `blackness` getters, and a `changeHwb()` method.

[HWB]: https://en.wikipedia.org/wiki/HWB_color_model