Switch from AsyncThunk to AsyncMemoizer.

lib/src/util/async_thunk.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 test.util.async_thunk;
-
-import 'dart:async';
-
-/// A class for running an asynchronous method body exactly once and caching its
-/// result.
-///
-/// This should be stored as an instance variable, and [run] should be called
-/// when the method is invoked with the uncached method body. The first time, it
-/// runs the body; after that, it returns the future from the first run.
-class AsyncThunk<T> {
-  /// The completer for the method's result.
-  ///
-  /// This will be `null` if [run] hasn't been called yet.
-  Completer<T> _completer;
-
-  /// Whether [run] has been called yet.
-  bool get hasRun => _completer != null;
-
-  /// Runs the method body, [fn], if it hasn't been run before.
-  ///
-  /// If [fn] has been run before, returns the original result.
-  Future<T> run(fn()) {
-    if (_completer == null) {
-      _completer = new Completer.sync();
-      new Future.sync(fn)
-          .then(_completer.complete)
-          .catchError(_completer.completeError);
-    }
-
-    return _completer.future;
-  }
-}