Dart Sass 1.0.0-alpha.6

• Allow var() to be passed to rgb(), rgba(), hsl(), and hsla().
• Fix conversions between numbers with dpi, dpcm, and dppx units. Previously these conversions were inverted.
• Don't crash when calling str-slice() with an $end-at index lower than the $start-at index.
• str-slice() now correctly returns "" when $end-at is negative and points before the beginning of the string.
• Interpolation in quoted strings now properly preserves newlines.
• Don't crash when passing only $hue or no keyword arguments to adjust-color(), scale-color(), or change-color().
• Preserve escapes in identifiers. This used to only work for identifiers in SassScript.
• Fix a few small @extend bugs.