Refactor groups to pipe them through to the runner.

lib/src/backend/suite_entry.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 test.backend.test_or_group;
+
+import 'metadata.dart';
+import 'operating_system.dart';
+import 'test.dart';
+import 'test_platform.dart';
+
+/// A [Test] or [Group].
+abstract class SuiteEntry {
+  /// The name of the entry, includes the prefixes from any containing [Group]s.
+  String get name;
+
+  /// The metadata for the entry, including the metadata from any containing
+  /// [Group]s and the test suite.
+  Metadata get metadata;
+
+  /// Returns a copy of [this] with all platform-specific metadata resolved.
+  ///
+  /// Removes any tests and groups with [Metadata.testOn] selectors that don't
+  /// match [platform] and [selector]. Returns `null` if this entry's selector
+  /// doesn't match.
+  SuiteEntry forPlatform(TestPlatform platform, {OperatingSystem os});
+
+  /// Returns a copy of [this] with all tests that don't match [callback]
+  /// removed.
+  ///
+  /// Returns `null` if this is a test that doesn't match [callback] or a group
+  /// where no child tests match [callback].
+  SuiteEntry filter(bool callback(Test test));
+}