Add support for evaluating platform selectors.

lib/src/backend/platform_selector/evaluator.dart

+// Copyright (c) 2015, 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.
+
+library unittest.backend.platform_selector.evaluator;
+
+import 'package:source_span/source_span.dart';
+
+import '../operating_system.dart';
+import '../test_platform.dart';
+import 'ast.dart';
+import 'visitor.dart';
+
+/// A visitor for evaluating platform selectors against a specific
+/// [TestPlatform] and [OperatingSystem].
+class Evaluator implements Visitor<bool> {
+  /// The platform to test against.
+  final TestPlatform _platform;
+
+  /// The operating system to test against.
+  final OperatingSystem _os;
+
+  Evaluator(this._platform, {OperatingSystem os})
+      : _os = os == null ? OperatingSystem.none : os;
+
+  bool visitVariable(VariableNode node) {
+    if (node.name == _platform.identifier) return true;
+    if (node.name == _os.name) return true;
+
+    switch (node.name) {
+      case "dart-vm": return _platform.isDartVm;
+      case "browser": return _platform.isBrowser;
+      case "js": return _platform.isJS;
+      case "blink": return _platform.isBlink;
+      case "posix": return _os.isPosix;
+      default: return false;
+    }
+  }
+
+  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);
+}