Update charted to 0.4.8 and roll

packages/async/lib/src/async_memoizer.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.async_memoizer;
-
 import 'dart:async';
 
 /// A class for running an asynchronous function exactly once and caching its
 /// result.
 ///
 /// An `AsyncMemoizer` is used when some function may be run multiple times in
 /// order to get its result, but it only actually needs to be run once for its
 /// effect. To memoize the result of an async function, you can create a
 /// memoizer outside the function (for example as an instance field if you want
 /// to memoize the result of a method), and then wrap the function's body in a
 /// call to [runOnce].
 ///
 /// This is useful for methods like `close()` and getters that need to do
 /// asynchronous work. For example:
 ///
 /// ```dart
 /// class SomeResource {
 ///   final _closeMemo = new AsyncMemoizer();
 ///
 ///   Future close() => _closeMemo.runOnce(() {
 ///     // ...
 ///   });
 /// }
 /// ```
 class AsyncMemoizer<T> {
   /// The future containing the method's result.
   ///
   /// This can be accessed at any time, and will fire once [runOnce] is called.
   Future<T> get future => _completer.future;
-  final _completer = new Completer();
+  final _completer = new Completer<T>();
 
-  /// Whether [run] has been called yet.
+  /// Whether [runOnce] has been called yet.
   bool get hasRun => _completer.isCompleted;
 
   /// Runs the function, [computation], if it hasn't been run before.
   ///
-  /// If [run] has already been called, this returns the original result.
-  Future<T> runOnce(computation()) {
+  /// If [runOnce] has already been called, this returns the original result.
+  Future<T> runOnce(FutureOr<T> computation()) {
     if (!hasRun) _completer.complete(new Future.sync(computation));
     return future;
   }
 }