Add an AsyncThunk class.

test/async_memoizer_test.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';
+
+import 'package:async/async.dart';
+import 'package:test/test.dart';
+
+main() {
+  var cache;
+  setUp(() => cache = new AsyncMemoizer());
+
+  test("runs the function only the first time runOnce() is called", () async {
+    var count = 0;
+    expect(await cache.runOnce(() => count++), equals(0));
+    expect(count, equals(1));
+
+    expect(await cache.runOnce(() => count++), equals(0));
+    expect(count, equals(1));
+  });
+
+  test("forwards the return value from the function", () async {
+    expect(cache.runOnce(() => "value"), completion(equals("value")));
+    expect(cache.runOnce(() {}), completion(equals("value")));
+  });
+
+  test("forwards the return value from an async function", () async {
+    expect(cache.runOnce(() async => "value"), completion(equals("value")));
+    expect(cache.runOnce(() {}), completion(equals("value")));
+  });
+
+  test("forwards the error from an async function", () async {
+    expect(cache.runOnce(() async => throw "error"), throwsA("error"));
+    expect(cache.runOnce(() {}), throwsA("error"));
+  });
+}