Add a scanner and parser for platform selectors.

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.
+
+library unittest.backend.platform_selector.ast;
+
+import 'package:source_span/source_span.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;
+}
+
+/// A single variable.
+class VariableNode implements Node {
+  final FileSpan span;
+
+  /// The variable name.
+  final String name;
+
+  VariableNode(this.name, [this.span]);
+
+  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]);
+
+  String toString() => child is VariableNode || child is NotNode
+      ? "!$child"
+      : "!($child)";
+}
+
+/// An or expression.
+class OrNode implements Node {
+  FileSpan get span => _expandSafe(branch1.span, branch2.span);
+
+  /// The left-hand branch of the expression.
+  final Node branch1;

[Bob Nystrom, 2015/03/11 20:05:52]

How about "left" and "right" for these?

[nweiz, 2015/03/12 19:48:57]

Done.

+
+  /// The right-hand branch of the expression.
+  final Node branch2;
+
+  OrNode(this.branch1, this.branch2);
+
+  String toString() {
+    var string1 = branch1 is AndNode || branch1 is ConditionalNode
+        ? "($branch1)"
+        : branch1;
+    var string2 = branch2 is AndNode || branch2 is ConditionalNode
+        ? "($branch2)"
+        : branch2;
+
+    return "$string1 || $string2";
+  }
+}
+
+/// An and expression.
+class AndNode implements Node {
+  FileSpan get span => _expandSafe(branch1.span, branch2.span);
+
+  /// The left-hand branch of the expression.
+  final Node branch1;
+
+  /// The right-hand branch of the expression.
+  final Node branch2;
+
+  AndNode(this.branch1, this.branch2);
+
+  String toString() {
+    var string1 = branch1 is OrNode || branch1 is ConditionalNode
+        ? "($branch1)"
+        : branch1;
+    var string2 = branch2 is OrNode || branch2 is ConditionalNode
+        ? "($branch2)"
+        : branch2;
+
+    return "$string1 && $string2";
+  }
+}
+
+/// A ternary conditional expression.
+class ConditionalNode implements Node {
+  FileSpan get span => _expandSafe(condition.span, branch2.span);
+
+  /// The condition expression to check.
+  final Node condition;
+
+  /// The branch to run if the condition is true.
+  final Node branch1;

[Bob Nystrom, 2015/03/11 20:05:52]

"thenBranch" and "elseBranch"?

[nweiz, 2015/03/12 19:48:57]

Decided to go with "whenTrue" and "whenFalse" instead.

+
+  /// The branch to run if the condition is false.
+  final Node branch2;
+
+  ConditionalNode(this.condition, this.branch1, this.branch2);
+
+  String toString() {
+    var conditionString =
+        condition is ConditionalNode ? "($condition)" : condition;
+    var string1 = branch1 is ConditionalNode ? "($branch1)" : branch1;
+    return "$conditionString ? $string1 : $branch2";
+  }
+}
+
+/// 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);
+}