Add support for evaluating platform selectors.

lib/src/backend/platform_selector/ast.dart

Index: lib/src/backend/platform_selector/ast.dart
diff --git a/lib/src/backend/platform_selector/ast.dart b/lib/src/backend/platform_selector/ast.dart
index d1eb0aac16f50b30f33a9256e171cb48dd405313..c76fe18096598fa13056f6b41525607b6b3e461f 100644
--- a/lib/src/backend/platform_selector/ast.dart
+++ b/lib/src/backend/platform_selector/ast.dart
@@ -6,6 +6,8 @@ library unittest.backend.platform_selector.ast;
 
 import 'package:source_span/source_span.dart';
 
+import 'visitor.dart';
+
 /// The superclass of nodes in the platform selector abstract syntax tree.
 abstract class Node {
   /// The span indicating where this node came from.
@@ -16,6 +18,9 @@ abstract class Node {
   ///
   /// This may be `null` for nodes without source information.
   FileSpan get span;
+
+  /// Calls the appropriate [Visitor] method on [this] and returns the result.
+  accept(Visitor visitor);
 }
 
 /// A single variable.
@@ -27,6 +32,8 @@ class VariableNode implements Node {
 
   VariableNode(this.name, [this.span]);
 
+  accept(Visitor visitor) => visitor.visitVariable(this);
+
   String toString() => name;
 }
 
@@ -39,6 +46,8 @@ class NotNode implements Node {
 
   NotNode(this.child, [this.span]);
 
+  accept(Visitor visitor) => visitor.visitNot(this);
+
   String toString() => child is VariableNode || child is NotNode
       ? "!$child"
       : "!($child)";
@@ -56,6 +65,8 @@ class OrNode implements Node {
 
   OrNode(this.left, this.right);
 
+  accept(Visitor visitor) => visitor.visitOr(this);
+
   String toString() {
     var string1 = left is AndNode || left is ConditionalNode
         ? "($left)"
@@ -80,6 +91,8 @@ class AndNode implements Node {
 
   AndNode(this.left, this.right);
 
+  accept(Visitor visitor) => visitor.visitAnd(this);
+
   String toString() {
     var string1 = left is OrNode || left is ConditionalNode
         ? "($left)"
@@ -107,6 +120,8 @@ class ConditionalNode implements Node {
 
   ConditionalNode(this.condition, this.whenTrue, this.whenFalse);
 
+  accept(Visitor visitor) => visitor.visitConditional(this);
+
   String toString() {
     var conditionString =
         condition is ConditionalNode ? "($condition)" : condition;