Add GroupSet and SetGroup classes.

lib/src/set_group.dart

Index: lib/src/set_group.dart
diff --git a/lib/src/set_group.dart b/lib/src/set_group.dart
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+++ b/lib/src/set_group.dart
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+// Copyright (c) 2016, 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.
+
+import 'group_set.dart';
+import 'wrappers.dart';
+
+/// A set of sets that provides [set], a view of the union of those sets.
+///
+/// This is a convenience class for creating a [GroupSet] whose contents change
+/// over the lifetime of a class. For example:
+///
+/// ```dart
+/// class Foo {
+///   Set<String> get contents => _contentsGroup.set;
+///   final _contentsGroup = new SetGroup<String>();
+///
+///   // ...
+/// }
+/// ```
+class SetGroup<E> extends DelegatingSet<Set<E>> {
+  /// The [GroupSet] that provides a view of the union of sets in [this].
+  GroupSet<E> get set => _set;
+  GroupSet<E> _set;
+
+  /// Creates a set of sets that provides a view of the union of those sets.
+  ///
+  /// If [disjoint] is `true`, this assumes that all component sets are
+  /// disjoint—that is, that they contain no elements in common. This makes
+  /// many operations including [length] more efficient.
+  SetGroup({bool disjoint: false}) : super(new Set<Set<E>>()) {
+    _set = new GroupSet<E>(this, disjoint: disjoint);
+  }
+}