Track missing alpha channels in colors

See sass/sass#2831

Author: nex3

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);