Define equality for boolean selectors.

lib/src/union_selector.dart

Index: lib/src/union_selector.dart
diff --git a/lib/src/union_selector.dart b/lib/src/union_selector.dart
index 6b6cf87dad04f512c19a1beb8f5713529f82a9d2..f513f6c1db22ea65611728054c4745fe95d6cd85 100644
--- a/lib/src/union_selector.dart
+++ b/lib/src/union_selector.dart
@@ -30,4 +30,11 @@ class UnionSelector implements BooleanSelector {
   }
 
   String toString() => "($_selector1) && ($_selector2)";
+
+  bool operator==(other) =>
+      other is UnionSelector &&
+      _selector1 == other._selector1 &&
+      _selector2 == other._selector2;
+
+  int get hashCode => _selector1.hashCode ^ _selector2.hashCode;
 }