Make `TypeSafeStream` extend `Stream`.

lib/src/typed/stream.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 'dart:async';
 
 import 'package:collection/collection.dart';
 
 import '../utils.dart';
 import 'stream_subscription.dart';
 import '../delegate/event_sink.dart';
 
-class TypeSafeStream<T> implements Stream<T> {
+class TypeSafeStream<T> extends Stream<T> {

[floitsch, 2017/05/16 09:01:18]

should we add `groupBy` as an optimization?

   final Stream _stream;
 
   Future<T> get first async => (await _stream.first) as T;
   Future<T> get last async => (await _stream.last) as T;
   Future<T> get single async => (await _stream.single) as T;
 
   bool get isBroadcast => _stream.isBroadcast;
   Future<bool> get isEmpty => _stream.isEmpty;
   Future<int> get length => _stream.length;
 
@@ skipping 98 matching lines @@
   Future<T> elementAt(int index) async => (await _stream.elementAt(index)) as T;
   Future<bool> contains(Object needle) => _stream.contains(needle);
   Future<String> join([String separator = ""]) => _stream.join(separator);
   String toString() => _stream.toString();
 
   /// Returns a version of [function] that asserts that its argument is an
   /// instance of `T`.
   UnaryFunction<dynamic, S> _validateType<S>(S function(T value)) =>
       function == null ? null : (value) => function(value as T);
 }