Add support for evaluating platform selectors.

lib/src/backend/platform_selector.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;
+
+import 'package:source_span/source_span.dart';
+
+import 'operating_system.dart';
+import 'platform_selector/ast.dart';
+import 'platform_selector/evaluator.dart';
+import 'platform_selector/parser.dart';
+import 'platform_selector/visitor.dart';
+import 'test_platform.dart';
+
+/// The set of all valid variable names.
+final _validVariables =
+    new Set<String>.from(["posix", "dart-vm", "browser", "dart2js", "blink"])
+        ..addAll(TestPlatform.all.map((platform) => platform.identifier))
+        ..addAll(OperatingSystem.all.map((os) => os.name));
+
+/// An expressing for selecting certain platforms, including operating systems

[Bob Nystrom, 2015/03/16 16:54:26]

"expressing" -> "expression".

[nweiz, 2015/03/24 01:03:28]

Done.

+/// and browsers.
+///
+/// The syntax is mostly Dart's expression syntax restricted to boolean
+/// operations. See the README for full details.
+class PlatformSelector {
+  /// The parsed AST.
+  final Node _selector;
+
+  /// Parses [selector].
+  ///
+  /// This will throw a [SourceSpanFormatException] if the selector is
+  /// malformed or if it uses an undefined variable.
+  PlatformSelector.parse(String selector)
+      : _selector = new Parser(selector).parse() {
+    _selector.accept(const _VariableValidator());
+  }
+
+  /// Evaluates the selector against the given [platform] and [os].

[Bob Nystrom, 2015/03/16 16:54:26]

How about something that explains what the evaluation does. Maybe like "Returns
`true` if the selector matches [platform] and [os]."

[nweiz, 2015/03/24 01:03:28]

Done.

+  ///
+  /// [os] defaults to [OperatingSystem.none].
+  bool evaluate(TestPlatform platform, {OperatingSystem os}) =>
+      _selector.accept(new Evaluator(platform, os: os));
+}
+
+/// An AST visitor that ensures that all variables are valid.
+///
+/// This isn't done when evaluating to ensure that errors are eagerly detected,
+/// and it isn't done when parsing to avoid coupling the syntax too tightly to
+/// the semantics.
+class _VariableValidator extends RecursiveVisitor {
+  const _VariableValidator();
+
+  void visitVariable(VariableNode node) {
+    if (_validVariables.contains(node.name)) return;
+    throw new SourceSpanFormatException("Undefined variable.", node.span);
+  }
+}