Add a testOn parameter to test() and group().

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';
@@ skipping 26 matching lines @@
   ///
   /// This will throw a [SourceSpanFormatException] if the selector is
   /// malformed or if it uses an undefined variable.
   factory PlatformSelector.parse(String selector) =>
       new _PlatformSelector.parse(selector);
 
   /// Returns whether the selector matches the given [platform] and [os].
   ///
   /// [os] defaults to [OperatingSystem.none].
   bool evaluate(TestPlatform platform, {OperatingSystem os});
+
+  /// Returns a new [PlatformSelector] that matches only platforms matched by
+  /// both [this] and [other].
+  PlatformSelector intersect(PlatformSelector other);
 }
 
 /// The concrete implementation of a [PlatformSelector] parsed from a string.
 ///
 /// This is separate from [PlatformSelector] so that [_AllPlatforms] can
 /// implement [PlatformSelector] without having to implement private members.
 class _PlatformSelector implements PlatformSelector{
   /// The parsed AST.
   final Node _selector;
 
   _PlatformSelector.parse(String selector)
       : _selector = new Parser(selector).parse() {
     _selector.accept(const _VariableValidator());
   }
 
   _PlatformSelector(this._selector);
 
   bool evaluate(TestPlatform platform, {OperatingSystem os}) =>
       _selector.accept(new Evaluator(platform, os: os));
 
+  PlatformSelector intersect(PlatformSelector other) {
+    if (other == PlatformSelector.all) return this;
+    return new _PlatformSelector(new AndNode(
+        _selector, (other as _PlatformSelector)._selector));
+  }
+
   String toString() => _selector.toString();
 }
 
 /// A selector that matches all platforms.
 class _AllPlatforms implements PlatformSelector {
   const _AllPlatforms();
 
   bool evaluate(TestPlatform platform, {OperatingSystem os}) => true;
 
+  PlatformSelector intersect(PlatformSelector other) => other;
+
   String toString() => "*";
 }
 
 /// 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);
   }
 }