Add an AsyncThunk class.

lib/src/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 async.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.
+///
+/// This is useful for methods like `close()` and getters that need to do
+/// asynchronous work. For example:
+///
+/// ```dart
+/// class SomeResource {
+///   final _closeThunk = new AsyncThunk();
+///
+///   Future close() {
+///     return _closeThunk.run(() {
+///       // ...
+///     });
+///   }
+/// }
+/// ```
+class AsyncThunk<T> {

[Lasse Reichstein Nielsen, 2015/07/01 05:23:18]

This can all just be: 
  
class AsyncThunk<T> {
  Future<T> _future;
  Future<T> run(fn()) {
    if (_future == null) _future = new Future<T>.sync(fn);
    return _future;
  }
}

That doesn't really depend on the cached value being a future, so it could just
be:
  class InitOnce {
    var _value; 
    run(fn()) {
      // maybe a little more complexity to handle fn returning null.
      if (_value == null) _value = fn();
      return _value;
    }
  }

that you call with:
  _initOnce.run(() => new Future<T>.sync(fn));
instead of 
  _closeThunk.run(fn);


So, I'm not entirely sure I think this class carries its own weight. Introducing
a class means introducing a concept that the user needs to understand and
remember. The concept here is so basic that it might be causing more confusion
to introduce a new name for it than can be outweighed by the complexity it
reduces when using it.

[nweiz, 2015/07/01 19:07:23]

Done.
You're right that this class is simple to implement manually, but the fact that
it introduces a concept in its own right is part of why I think it's important
to have. It provides a single consistent way of doing a common pattern—for users
who use this pattern a lot (generally application authors), being able to name
it lowers the conceptual burden rather than increasing it.

The test package, where I created this class originally, is a great example: it
has a bunch of components that need close methods that behave a certain way, and
having a clearly visible indicator of that makes the code easier to follow.

+  /// 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 [run] has already been called, this 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;
+  }
+}