Add GroupSet and SetGroup classes.

lib/src/union_set_controller.dart

Index: lib/src/union_set_controller.dart
diff --git a/lib/src/union_set_controller.dart b/lib/src/union_set_controller.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 'union_set.dart';
+
+/// A controller that exposes a view of the union of a collection of sets.
+///
+/// This is a convenience class for creating a [UnionSet] whose contents change
+/// over the lifetime of a class. For example:
+///
+/// ```dart
+/// class Engine {
+///   Set<Test> get activeTests => _activeTestsGroup.set;
+///   final _activeTestsGroup = new UnionSetController<Test>();
+///
+///   void addSuite(Suite suite) {
+///     _activeTestsGroup.add(suite.tests);
+///     _runSuite(suite);
+///     _activeTestsGroup.remove(suite.tests);
+///   }
+/// }
+/// ```
+class UnionSetController<E> {
+  /// The [UnionSet] that provides a view of the union of sets in [this].
+  UnionSet<E> get set => _set;
+  UnionSet<E> _set;
+
+  /// The sets whose union is exposed through [set].
+  final _sets = new Set<Set<E>>();
+
+  /// 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.
+  UnionSetController({bool disjoint: false}) {
+    _set = new UnionSet<E>(_sets, disjoint: disjoint);
+  }
+
+  /// Adds the contents of [component] to [set].
+  ///
+  /// If the contents of [component] change over time, [set] will change
+  /// accordingly.
+  void add(Set<E> component) {
+    _sets.add(component);
+  }
+
+  /// Removes the contents of [component] to [set].
+  ///
+  /// If another set in [this] has overlapping elements with [component], those
+  /// elements will remain in [set].
+  bool remove(Set<E> component) => _sets.remove(component);
+}