Make PlatformSelector use boolean_selector.

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.
-
-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);
-}