Add an AsyncThunk class.

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