Add typed wrapper functions to delegate classes.

lib/src/delegate/stream_consumer.dart

 // Copyright (c) 2015, 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';
 
 /// Simple delegating wrapper around a [StreamConsumer].
 ///
 /// Subclasses can override individual methods, or use this to expose only the
 /// [StreamConsumer] methods of a subclass.
 class DelegatingStreamConsumer<T> implements StreamConsumer<T> {
   final StreamConsumer _consumer;
 
   /// Create a delegating consumer forwarding calls to [consumer].
-  DelegatingStreamConsumer(StreamConsumer consumer)
-      : _consumer = consumer;
+  DelegatingStreamConsumer(StreamConsumer<T> consumer) : _consumer = consumer;
+
+  DelegatingStreamConsumer._(this._consumer);
+
+  /// Creates a wrapper that coerces the type of [consumer].
+  ///
+  /// Unlike [new StreamConsumer], this only requires its argument to be an
+  /// instance of `StreamConsumer`, not `StreamConsumer<T>`. This means that
+  /// calls to [addStream] may throw a [CastError] if the argument type doesn't
+  /// match the reified type of [consumer].
+  static StreamConsumer/*<T>*/ typed/*<T>*/(StreamConsumer consumer) =>
+      consumer is StreamConsumer/*<T>*/
+          ? consumer
+          : new DelegatingStreamConsumer._(consumer);
 
   Future addStream(Stream<T> stream) => _consumer.addStream(stream);
 
   Future close() => _consumer.close();
 }