Add the content of the package.

lib/src/evaluator.dart

Index: lib/src/evaluator.dart
diff --git a/lib/src/evaluator.dart b/lib/src/evaluator.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 'ast.dart';
+import 'visitor.dart';
+
+typedef bool _Semantics(String variable);
+
+/// A visitor for evaluating boolean selectors against a specific set of
+/// semantics.
+class Evaluator implements Visitor<bool> {
+  /// The semantics to evaluate against.
+  final _Semantics _semantics;
+
+  Evaluator(semantics)
+      : _semantics = semantics is Iterable
+            ? semantics.toSet().contains
+            : semantics;
+
+  bool visitVariable(VariableNode node) => _semantics(node.name);
+
+  bool visitNot(NotNode node) => !node.child.accept(this);
+
+  bool visitOr(OrNode node) =>
+      node.left.accept(this) || node.right.accept(this);
+
+  bool visitAnd(AndNode node) =>
+      node.left.accept(this) && node.right.accept(this);
+
+  bool visitConditional(ConditionalNode node) => node.condition.accept(this)
+      ? node.whenTrue.accept(this)
+      : node.whenFalse.accept(this);
+}