Add support for evaluating platform selectors.

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.
+
+library unittest.backend.platform_selector.visitor;
+
+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);
+  }
+}