Add a LiveSuite class.

lib/src/util/iterable_set.dart

Index: lib/src/util/iterable_set.dart
diff --git a/lib/src/util/iterable_set.dart b/lib/src/util/iterable_set.dart
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+++ b/lib/src/util/iterable_set.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 'dart:collection';
+
+import 'package:collection/collection.dart';
+
+/// An unmodifiable [Set] view backed by an arbitrary [Iterable].
+///
+/// Note that contrary to most APIs that take iterables, this does not convert
+/// its argument to another collection before use. This means that if it's
+/// lazily-generated, that generation will happen for every operation.
+///
+/// Note also that set operations that are usually expected to be `O(1)` or
+/// `O(log(n))`, such as [contains], may be `O(n)` for many underlying iterable
+/// types. As such, this should only be used for small iterables.
+class IterableSet<E> extends SetMixin<E> with UnmodifiableSetMixin<E> {
+  /// The base iterable that set operations forward to.
+  final Iterable<E> _base;
+
+  int get length => _base.length;
+
+  Iterator<E> get iterator => _base.iterator;
+
+  /// Creates a [Set] view of [base].
+  IterableSet(this._base);
+
+  bool contains(Object element) => _base.contains(element);
+
+  E lookup(Object needle) =>
+      _base.firstWhere((element) => element == needle, orElse: () => null);
+
+  Set<E> toSet() => _base.toSet();
+}