Fix universal selector unification.

See sass/sass#2247

README.md

@@ -178,6 +178,8 @@ Sass to update the reference behavior.
 
 13. The reference combinator is not supported. See [issue 303][].
 
+14. Universal selector unification is symmetrical. See [issue 2247][].
+
 [issue 1599]: https://github.com/sass/sass/issues/1599
 [issue 1126]: https://github.com/sass/sass/issues/1126
 [issue 2120]: https://github.com/sass/sass/issues/2120
@@ -191,5 +193,6 @@ Sass to update the reference behavior.
 [issue 2228]: https://github.com/sass/sass/issues/2228
 [issue 2245]: https://github.com/sass/sass/issues/2245
 [issue 303]: https://github.com/sass/sass/issues/303
+[issue 2247]: https://github.com/sass/sass/issues/2247
 
 Disclaimer: this is not an official Google product.

lib/src/ast/selector/pseudo.dart

@@ -84,6 +84,9 @@ class PseudoSelector extends SimpleSelector {
   }
 
   List<SimpleSelector> unify(List<SimpleSelector> compound) {
+    if (compound.length == 1 && compound.first is UniversalSelector) {
+      return compound.first.unify([this]);
+    }
     if (compound.contains(this)) return compound;
 
     var result = <SimpleSelector>[];

lib/src/ast/selector/simple.dart

@@ -56,6 +56,9 @@ abstract class SimpleSelector extends Selector {
   /// Returns `null` if unification is impossible—for example, if there are
   /// multiple ID selectors.
   List<SimpleSelector> unify(List<SimpleSelector> compound) {
+    if (compound.length == 1 && compound.first is UniversalSelector) {
+      return compound.first.unify([this]);
+    }
     if (compound.contains(this)) return compound;
 
     var result = <SimpleSelector>[];