Update charted to 0.4.8 and roll

packages/async/lib/src/delegate/event_sink.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.
 
-library async.delegate.event_sink;
-
 import 'dart:async';
 
 /// Simple delegating wrapper around an [EventSink].
 ///
 /// Subclasses can override individual methods, or use this to expose only the
 /// [EventSink] methods of a subclass.
 class DelegatingEventSink<T> implements EventSink<T> {
   final EventSink _sink;
 
   /// Create a delegating sink forwarding calls to [sink].
-  DelegatingEventSink(EventSink sink) : _sink = sink;
+  DelegatingEventSink(EventSink<T> sink) : _sink = sink;
+
+  DelegatingEventSink._(this._sink);
+
+  /// Creates a wrapper that coerces the type of [sink].
+  ///
+  /// Unlike [new DelegatingEventSink], this only requires its argument to be an
+  /// instance of `EventSink`, not `EventSink<T>`. This means that calls to
+  /// [add] may throw a [CastError] if the argument type doesn't match the
+  /// reified type of [sink].
+  static EventSink<T> typed<T>(EventSink sink) =>
+      sink is EventSink<T> ? sink : new DelegatingEventSink._(sink);
 
   void add(T data) {
     _sink.add(data);
   }
 
   void addError(error, [StackTrace stackTrace]) {
     _sink.addError(error, stackTrace);
   }
 
   void close() {
     _sink.close();
   }
 }