Add the content of the package.

lib/src/impl.dart

Index: lib/src/impl.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 '../boolean_selector.dart';
+import 'ast.dart';
+import 'evaluator.dart';
+import 'intersection_selector.dart';
+import 'parser.dart';
+import 'union_selector.dart';
+import 'validator.dart';
+
+/// The concrete implementation of a [BooleanSelector] parsed from a string.
+///
+/// This is separate from [BooleanSelector] so that [intersect] and [union] can
+/// check to see whether they're passed a [BooleanSelectorImpl] or a different
+/// class that implements [BooleanSelector].
+class BooleanSelectorImpl implements BooleanSelector {
+  /// The parsed AST.
+  final Node _selector;
+
+  /// Parses [selector].
+  ///
+  /// This will throw a [SourceSpanFormatException] if the selector is
+  /// malformed or if it uses an undefined variable.
+  BooleanSelectorImpl.parse(String selector)
+      : _selector = new Parser(selector).parse();
+
+  BooleanSelectorImpl._(this._selector);
+
+  Iterable<String> get variables => _selector.variables;
+
+  bool evaluate(semantics) => _selector.accept(new Evaluator(semantics));
+
+  BooleanSelector intersection(BooleanSelector other) {
+    if (other == BooleanSelector.all) return this;
+    if (other == BooleanSelector.none) return other;
+    return other is BooleanSelectorImpl
+        ? new BooleanSelectorImpl._(new AndNode(_selector, other._selector))
+        : new IntersectionSelector(this, other);
+  }
+
+  BooleanSelector union(BooleanSelector other) {
+    if (other == BooleanSelector.all) return other;
+    if (other == BooleanSelector.none) return this;
+    return other is BooleanSelectorImpl
+        ? new BooleanSelectorImpl._(new OrNode(_selector, other._selector))
+        : new UnionSelector(this, other);
+  }
+
+  void validate(bool isDefined(String variable)) {
+    _selector.accept(new Validator(isDefined));
+  }
+
+  String toString() => _selector.toString();
+}