Add the content of the package.

lib/boolean_selector.dart

Index: lib/boolean_selector.dart
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+// 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
+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;
+
+  /// Parses [selector].
+  ///
+  /// This will throw a [SourceSpanFormatException] if the selector is
+  /// malformed or if it uses an undefined variable.
+  factory BooleanSelector.parse(String selector) = BooleanSelectorImpl.parse;
+
+  /// Returns whether the selector matches the given [semantics].
+  ///
+  /// The [semantics] define which variables evaluate to `true` or `false`. The
+  /// parameter can be either an `Iterable<String>` containing variables that
+  /// should evaluate to `true`, or a function `bool semantics(String variable)`
+  /// that returns a variable's value.
+  bool evaluate(semantics);
+
+  /// Returns a new [BooleanSelector] that matches only inputs matched by both
+  /// [this] and [other].
+  BooleanSelector intersection(BooleanSelector other);
+
+  /// 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));
+}