Add GroupSet and SetGroup classes.

test/set_group_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 group;
+  var innerSet;
+  setUp(() {
+    innerSet = new Set.from([1, 2, 3]);
+    group = new SetGroup<int>()..add(innerSet);
+  });
+
+  test("behaves like a set of sets", () {
+    expect(group, unorderedEquals([innerSet]));
+    expect(group, contains(innerSet));
+
+    var anotherSet = new Set.from([3, 4, 5]);
+    expect(group, isNot(contains(anotherSet)));
+
+    group.add(anotherSet);
+    expect(group, unorderedEquals([innerSet, anotherSet]));
+    expect(group, contains(anotherSet));
+  });
+
+  test("exposes a set group", () {
+    expect(group.set, unorderedEquals([1, 2, 3]));
+
+    group.add(new Set.from([3, 4, 5]));
+    expect(group.set, unorderedEquals([1, 2, 3, 4, 5]));
+
+    group.remove(innerSet);
+    expect(group.set, unorderedEquals([3, 4, 5]));
+  });
+}