Make PlatformSelector use boolean_selector.

lib/src/backend/platform_selector/ast.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 '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.
-  ///
-  /// This is a [FileSpan] because the nodes are parsed from a single continuous
-  /// string, but the string itself isn't actually a file. It might come from a
-  /// statically-parsed annotation or from a parameter.
-  ///
-  /// 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.
-class VariableNode implements Node {
-  final FileSpan span;
-
-  /// The variable name.
-  final String name;
-
-  VariableNode(this.name, [this.span]);
-
-  accept(Visitor visitor) => visitor.visitVariable(this);
-
-  String toString() => name;
-}
-
-/// A negation expression.
-class NotNode implements Node {
-  final FileSpan span;
-
-  /// The expression being negated.
-  final Node child;
-
-  NotNode(this.child, [this.span]);
-
-  accept(Visitor visitor) => visitor.visitNot(this);
-
-  String toString() => child is VariableNode || child is NotNode
-      ? "!$child"
-      : "!($child)";
-}
-
-/// An or expression.
-class OrNode implements Node {
-  FileSpan get span => _expandSafe(left.span, right.span);
-
-  /// The left-hand branch of the expression.
-  final Node left;
-
-  /// The right-hand branch of the expression.
-  final Node right;
-
-  OrNode(this.left, this.right);
-
-  accept(Visitor visitor) => visitor.visitOr(this);
-
-  String toString() {
-    var string1 = left is AndNode || left is ConditionalNode
-        ? "($left)"
-        : left;
-    var string2 = right is AndNode || right is ConditionalNode
-        ? "($right)"
-        : right;
-
-    return "$string1 || $string2";
-  }
-}
-
-/// An and expression.
-class AndNode implements Node {
-  FileSpan get span => _expandSafe(left.span, right.span);
-
-  /// The left-hand branch of the expression.
-  final Node left;
-
-  /// The right-hand branch of the expression.
-  final Node right;
-
-  AndNode(this.left, this.right);
-
-  accept(Visitor visitor) => visitor.visitAnd(this);
-
-  String toString() {
-    var string1 = left is OrNode || left is ConditionalNode
-        ? "($left)"
-        : left;
-    var string2 = right is OrNode || right is ConditionalNode
-        ? "($right)"
-        : right;
-
-    return "$string1 && $string2";
-  }
-}
-
-/// A ternary conditional expression.
-class ConditionalNode implements Node {
-  FileSpan get span => _expandSafe(condition.span, whenFalse.span);
-
-  /// The condition expression to check.
-  final Node condition;
-
-  /// The branch to run if the condition is true.
-  final Node whenTrue;
-
-  /// The branch to run if the condition is false.
-  final Node whenFalse;
-
-  ConditionalNode(this.condition, this.whenTrue, this.whenFalse);
-
-  accept(Visitor visitor) => visitor.visitConditional(this);
-
-  String toString() {
-    var conditionString =
-        condition is ConditionalNode ? "($condition)" : condition;
-    var trueString = whenTrue is ConditionalNode ? "($whenTrue)" : whenTrue;
-    return "$conditionString ? $trueString : $whenFalse";
-  }
-}
-
-/// Like [FileSpan.expand], except if [start] and [end] are `null` or from
-/// different files it returns `null` rather than throwing an error.
-FileSpan _expandSafe(FileSpan start, FileSpan end) {
-  if (start == null || end == null) return null;
-  if (start.file != end.file) return null;
-  return start.expand(end);
-}