Make PlatformSelector use boolean_selector.

lib/src/backend/platform_selector/visitor.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 'ast.dart';
-
-/// The interface for visitors of the platform selector AST.
-abstract class Visitor<T> {
-  T visitVariable(VariableNode node);
-  T visitNot(NotNode node);
-  T visitOr(OrNode node);
-  T visitAnd(AndNode node);
-  T visitConditional(ConditionalNode node);
-}
-
-/// An abstract superclass for side-effect-based visitors.
-///
-/// The default implementations of this visitor's methods just traverse the AST
-/// and do nothing with it.
-abstract class RecursiveVisitor implements Visitor {
-  const RecursiveVisitor();
-
-  void visitVariable(VariableNode node) {}
-
-  void visitNot(NotNode node) {
-    node.child.accept(this);
-  }
-
-  void visitOr(OrNode node) {
-    node.left.accept(this);
-    node.right.accept(this);
-  }
-
-  void visitAnd(AndNode node) {
-    node.left.accept(this);
-    node.right.accept(this);
-  }
-
-  void visitConditional(ConditionalNode node) {
-    node.condition.accept(this);
-    node.whenTrue.accept(this);
-    node.whenFalse.accept(this);
-  }
-}