Add typed wrapper functions to delegate classes.

lib/src/delegate/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 '../typed/stream.dart';
+
+/// Simple delegating wrapper around a [Stream].
+///
+/// Subclasses can override individual methods, or use this to expose only the
+/// [Stream] methods of a subclass.
+///
+/// Note that this is identical to [StreamView] in `dart:async`. It's provided
+/// under this name for consistency with other `Delegating*` classes.
+class DelegatingStream<T> extends StreamView<T> {
+  DelegatingStream(Stream<T> stream) : super(stream);
+
+  /// Creates a wrapper which throws if [stream]'s events aren't instances of
+  /// `T`.
+  ///
+  /// This soundly converts a [Stream] to a `Stream<T>`, regardless of its
+  /// original generic type, by asserting that its events are instances of `T`
+  /// whenever they're provided. If they're not, the stream throws a
+  /// [CastError].
+  static Stream/*<T>*/ typed/*<T>*/(Stream stream) =>
+      stream is Stream/*<T>*/ ? stream : new TypeSafeStream/*<T>*/(stream);
+}