[Color 4] Update color.same() and color equality (#2232)

See sass/sass#3852
See sass/sass#3858

Author: nex3

lib/src/functions/color.dart

@@ -504,12 +504,38 @@ final module = BuiltInModule("color", functions: <Callable>[
     var color1 = arguments[0].assertColor('color1');
     var color2 = arguments[1].assertColor('color2');
 
-    // Convert both colors into the same space to compare them. Usually we
-    // just use color1's space, but since HSL and HWB can't represent
-    // out-of-gamut colors we use RGB for all legacy color spaces.
-    var targetSpace = color1.isLegacy ? ColorSpace.rgb : color1.space;
-    return SassBoolean(
-        color1.toSpace(targetSpace) == color2.toSpace(targetSpace));
+    /// Converts [color] to the xyz-d65 space without any mising channels.
+    SassColor toXyzNoMissing(SassColor color) => switch (color) {
+          SassColor(space: ColorSpace.xyzD65, hasMissingChannel: false) =>
+            color,
+          SassColor(
+            space: ColorSpace.xyzD65,
+            :var channel0,
+            :var channel1,
+            :var channel2,
+            :var alpha
+          ) =>
+            SassColor.xyzD65(channel0, channel1, channel2, alpha),
+          SassColor(
+            :var space,
+            :var channel0,
+            :var channel1,
+            :var channel2,
+            :var alpha
+          ) =>
+            // Use [ColorSpace.convert] manually so that we can convert missing
+            // channels to 0 without having to create new intermediate color
+            // objects.
+            space.convert(
+                ColorSpace.xyzD65, channel0, channel1, channel2, alpha)
+        };
+
+    return SassBoolean(color1.space == color2.space
+        ? fuzzyEquals(color1.channel0, color2.channel0) &&
+            fuzzyEquals(color1.channel1, color2.channel1) &&
+            fuzzyEquals(color1.channel2, color2.channel2) &&
+            fuzzyEquals(color1.alpha, color2.alpha)
+        : toXyzNoMissing(color1) == toXyzNoMissing(color2));
   }),
 
   _function(

lib/src/util/number.dart

@@ -30,6 +30,17 @@ bool fuzzyEquals(num number1, num number2) {
           (number2 * _inverseEpsilon).round();
 }
 
+/// Like [fuzzyEquals], but allows null values for [number1] and [number2].
+///
+/// null values are only equal to one another.
+bool fuzzyEqualsNullable(num? number1, num? number2) {
+  if (number1 == number2) return true;
+  if (number1 == null || number2 == null) return false;
+  return (number1 - number2).abs() <= _epsilon &&
+      (number1 * _inverseEpsilon).round() ==
+          (number2 * _inverseEpsilon).round();
+}
+
 /// Returns a hash code for [number] that matches [fuzzyEquals].
 int fuzzyHashCode(double number) {
   if (!number.isFinite) return number.hashCode;

lib/src/value/color.dart

@@ -213,6 +213,16 @@ class SassColor extends Value {
         _ => true
       };
 
+  /// Whether this color has any missing channels.
+  ///
+  /// @nodoc
+  @internal
+  bool get hasMissingChannel =>
+      isChannel0Missing ||
+      isChannel1Missing ||
+      isChannel2Missing ||
+      isAlphaMissing;
+
   /// This color's red channel, between `0` and `255`.
   ///
   /// **Note:** This is rounded to the nearest integer, which may be lossy. Use
@@ -953,21 +963,21 @@ class SassColor extends Value {
 
     if (isLegacy) {
       if (!other.isLegacy) return false;
-      if (!fuzzyEquals(alpha, other.alpha)) return false;
-      if (space == ColorSpace.rgb && other.space == ColorSpace.rgb) {
-        return fuzzyEquals(channel0, other.channel0) &&
-            fuzzyEquals(channel1, other.channel1) &&
-            fuzzyEquals(channel2, other.channel2);
+      if (!fuzzyEqualsNullable(alphaOrNull, other.alphaOrNull)) return false;
+      if (space == other.space) {
+        return fuzzyEqualsNullable(channel0OrNull, other.channel0OrNull) &&
+            fuzzyEqualsNullable(channel1OrNull, other.channel1OrNull) &&
+            fuzzyEqualsNullable(channel2OrNull, other.channel2OrNull);
       } else {
         return toSpace(ColorSpace.rgb) == other.toSpace(ColorSpace.rgb);
       }
     }
 
     return space == other.space &&
-        fuzzyEquals(channel0, other.channel0) &&
-        fuzzyEquals(channel1, other.channel1) &&
-        fuzzyEquals(channel2, other.channel2) &&
-        fuzzyEquals(alpha, other.alpha);
+        fuzzyEqualsNullable(channel0OrNull, other.channel0OrNull) &&
+        fuzzyEqualsNullable(channel1OrNull, other.channel1OrNull) &&
+        fuzzyEqualsNullable(channel2OrNull, other.channel2OrNull) &&
+        fuzzyEqualsNullable(alphaOrNull, other.alphaOrNull);
   }
 
   int get hashCode {