Make Suite contain a single Group.

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));
-}