Add Declarer and Group classes.

lib/src/group.dart

Index: lib/src/group.dart
diff --git a/lib/src/group.dart b/lib/src/group.dart
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--- /dev/null
+++ b/lib/src/group.dart
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+// 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.group;
+
+import 'dart:async';
+
+import 'utils.dart';
+
+/// A group contains multiple tests and subgroups.
+///
+/// A group has a description that is prepended to that of all nested tests and
+/// subgroups. It also has [setUp] and [tearDown] functions which are scoped to
+/// the tests and groups it contains.
+class Group {
+  /// The parent group, or `null` if this is the root group.
+  final Group parent;
+
+  /// The description of the current test group, or `null` if this is the root
+  /// group.
+  final String _description;
+
+  /// The set-up function for this group, or `null`.
+  AsyncFunction setUp;
+
+  /// The tear-down function for this group, or `null`.
+  AsyncFunction tearDown;
+
+  /// Returns the description for this group, including the description of any
+  /// parent groups.
+  ///
+  /// If this is the root group, returns `null`.
+  String get description {
+    if (parent == null || parent.description == null) return _description;
+    return "${parent.description} $_description";
+  }
+
+  /// Creates a new root group.
+  ///
+  /// This is the implicit group that exists outside of any calls to `group()`.
+  Group.root()
+      : this(null, null);
+
+  Group(this.parent, this._description);
+
+  /// Run the set-up functions for this and any parent groups.
+  ///
+  /// If no set-up functions are declared, this returns a [Future] that
+  /// completes immediately.
+  Future runSetUp() {
+    if (parent != null) {
+      return parent.runSetUp().then((_) {
+        if (setUp != null) return setUp();
+      });
+    }
+
+    if (setUp != null) return new Future.sync(setUp);
+    return new Future.value();
+  }
+
+  /// Run the tear-up functions for this and any parent groups.
+  ///
+  /// If no set-up functions are declared, this returns a [Future] that
+  /// completes immediately.
+  Future runTearDown() {
+    if (parent != null) {
+      return new Future.sync(() {
+        if (tearDown != null) return tearDown();
+      }).then((_) => parent.runTearDown());
+    }
+
+    if (tearDown != null) return new Future.sync(tearDown);
+    return new Future.value();
+  }
+}