Track missing alpha channels in colors

See sass/sass#2831

lib/src/functions/color.dart

@@ -566,7 +566,7 @@ Value _invert(List<Value> arguments) {
  var rgb = color.toSpace(ColorSpace.rgb);
  return _mixLegacy(
  SassColor.rgb(255.0 - rgb.channel0, 255.0 - rgb.channel1,
- 255.0 - rgb.channel2, color.alpha),
+ 255.0 - rgb.channel2, color.alphaOrNull),
  color,
  weightNumber);
  }

lib/src/node/value/color.dart

@@ -6,18 +6,20 @@ import 'package:js/js.dart';
 
 import '../../value.dart';
 import '../reflection.dart';
+import '../utils.dart';
 
 /// The JavaScript `SassColor` class.
 final JSClass colorClass = () {
  var jsClass = createJSClass('sass.SassColor', (Object self, _Channels color) {
  if (color.red != null) {
- return SassColor.rgb(color.red!, color.green!, color.blue!, color.alpha);
+ return SassColor.rgb(color.red!, color.green!, color.blue!,
+ _handleUndefinedAlpha(color.alpha));
  } else if (color.saturation != null) {
- return SassColor.hsl(
- color.hue!, color.saturation!, color.lightness!, color.alpha);
+ return SassColor.hsl(color.hue!, color.saturation!, color.lightness!,
+ _handleUndefinedAlpha(color.alpha));
  } else {
- return SassColor.hwb(
- color.hue!, color.whiteness!, color.blackness!, color.alpha);
+ return SassColor.hwb(color.hue!, color.whiteness!, color.blackness!,
+ _handleUndefinedAlpha(color.alpha));
  }
  });
 
@@ -70,6 +72,12 @@ final JSClass colorClass = () {
  return jsClass;
 }();
 
+/// Converts an undefined [alpha] to 1.
+///
+/// This ensures that an explicitly null alpha will be treated as a missing
+/// component.
+num? _handleUndefinedAlpha(num? alpha) => isUndefined(alpha) ? 1 : alpha;
+
 @JS()
 @anonymous
 class _Channels {

lib/src/parse/stylesheet.dart

@@ -2433,7 +2433,7 @@ abstract class StylesheetParser extends Parser {
  red,
  green,
  blue,
- alpha,
+ alpha ?? 1,
  // Don't emit four- or eight-digit hex colors as hex, since that's not
  // yet well-supported in browsers.
  alpha == null ? SpanColorFormat(scanner.spanFrom(start)) : null);