Make Suite contain a single Group.

lib/src/backend/group.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.group;
 
-import 'dart:collection';
-
 import 'metadata.dart';
 import 'operating_system.dart';
-import 'suite_entry.dart';
+import 'group_entry.dart';
 import 'test.dart';
 import 'test_platform.dart';
 
 /// A group contains one or more tests and subgroups.
 ///
 /// It includes metadata that applies to all contained tests.
-class Group implements SuiteEntry {
+class Group implements GroupEntry {
   final String name;
 
   final Metadata metadata;
 
   /// The children of this group.
-  final List<SuiteEntry> entries;
+  final List<GroupEntry> entries;
 
-  Group(this.name, this.metadata, Iterable<SuiteEntry> entries)
-      : entries = new UnmodifiableListView<SuiteEntry>(entries.toList());
+  /// Returns a new root-level group.
+  Group.root(Iterable<GroupEntry> entries, {Metadata metadata})
+      : this(null, entries, metadata: metadata);
+
+  Group(this.name, Iterable<GroupEntry> entries, {Metadata metadata})
+      : entries = new List<GroupEntry>.unmodifiable(entries),
+        metadata = metadata == null ? new Metadata() : metadata;
 
   Group forPlatform(TestPlatform platform, {OperatingSystem os}) {
     if (!metadata.testOn.evaluate(platform, os: os)) return null;
     var newMetadata = metadata.forPlatform(platform, os: os);
     var filtered = _map((entry) => entry.forPlatform(platform, os: os));
     if (filtered.isEmpty) return null;
-    return new Group(name, newMetadata, filtered);
+    return new Group(name, filtered, metadata: newMetadata);
   }
 
   Group filter(bool callback(Test test)) {
     var filtered = _map((entry) => entry.filter(callback));
     if (filtered.isEmpty) return null;
-    return new Group(name, metadata, filtered);
+    return new Group(name, filtered, metadata: metadata);
   }
 
   /// Returns the entries of this group mapped using [callback].
   ///
   /// Any `null` values returned by [callback] will be removed.
-  List<SuiteEntry> _map(SuiteEntry callback(SuiteEntry entry)) {
+  List<GroupEntry> _map(GroupEntry callback(GroupEntry entry)) {
     return entries
         .map((entry) => callback(entry))
         .where((entry) => entry != null)
         .toList();
   }
 }