Add an AsyncThunk class.

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 will be `null` if [run] hasn't been called yet.
+  Future<T> _future;
+
+  /// Whether [run] has been called yet.
+  bool get hasRun => _future != null;
+
+  /// 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 (_future == null) _future = new Future.sync(computation);
+    return _future;
+  }
+}