Define equality for boolean selectors.

lib/boolean_selector.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:source_span/source_span.dart';
 
 import 'src/all.dart';
 import 'src/impl.dart';
 import 'src/none.dart';
 
 /// A boolean expression that evaluates to `true` or `false` based on certain
 /// inputs.
 ///
 /// The syntax is mostly Dart's expression syntax restricted to boolean
 /// operations. See [the README][] for full details.
 ///
 /// [the README]: https://github.com/dart-lang/boolean_selector/blob/master/README.md
+///
+/// Boolean selectors support structural equality. Two selectors that have the
+/// same parsed structure are considered equal.
 abstract class BooleanSelector {
   /// A selector that accepts all inputs.
   static const all = const All();
 
   /// A selector that accepts no inputs.
   static const none = const None();
 
   /// All the variables in this selector, in the order they appear.
   Iterable<String> get variables;
 
@@ skipping 18 matching lines @@
   /// Returns a new [BooleanSelector] that matches all inputs matched by either
   /// [this] or [other].
   BooleanSelector union(BooleanSelector other);
 
   /// Throws a [FormatException] if any variables are undefined.
   ///
   /// The [isDefined] function should return `true` for any variables that are
   /// considered valid, and `false` for any invalid or undefined variables.
   void validate(bool isDefined(String variable));
 }