Add GroupSet and SetGroup classes.

test/union_set_controller_test.dart

+// 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 "package:test/test.dart";
+
+import "package:collection/collection.dart";
+
+void main() {
+  var controller;
+  var innerSet;
+  setUp(() {
+    innerSet = new Set.from([1, 2, 3]);
+    controller = new UnionSetController<int>()..add(innerSet);
+  });
+
+  test("exposes a union set", () {
+    expect(controller.set, unorderedEquals([1, 2, 3]));
+
+    controller.add(new Set.from([3, 4, 5]));
+    expect(controller.set, unorderedEquals([1, 2, 3, 4, 5]));
+
+    controller.remove(innerSet);
+    expect(controller.set, unorderedEquals([3, 4, 5]));
+  });
+
+  test("exposes a disjoint union set", () {
+    expect(controller.set, unorderedEquals([1, 2, 3]));
+
+    controller.add(new Set.from([4, 5, 6]));
+    expect(controller.set, unorderedEquals([1, 2, 3, 4, 5, 6]));
+
+    controller.remove(innerSet);
+    expect(controller.set, unorderedEquals([4, 5, 6]));
+  });
+}