Add built-in timeouts to scheduled_test.

pkg/scheduled_test/lib/src/substitute_future.dart

+// Copyright (c) 2013, 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 substitute_future;
+
+import 'dart:async';
+
+/// A wrapper for [Future] that allows other [Future]s to be substituted in as
+/// the wrapped [Future]. This is used for injecting timeout errors into
+/// long-running [Future]s.
+class SubstituteFuture<T> implements Future<T> {
+  /// The wrapped [Future].
+  Future<T> _inner;
+
+  /// The completer that corresponds to [this]'s result.
+  final Completer<T> _completer = new Completer<T>();
+
+  /// Whether or not [this] has been completed yet.
+  bool _complete = false;
+
+  SubstituteFuture(Future wrapped) {
+    substitute(wrapped);
+  }
+
+  Stream<T> asStream() => _completer.future.asStream();
+  Future catchError(onError(AsyncError asyncError), {bool test(error)}) =>
+    _completer.future.catchError(onError, test: test);
+  Future then(onValue(T value), {onError(AsyncError asyncError)}) =>
+    _completer.future.then(onValue, onError: onError);
+  Future<T> whenComplete(action()) => _completer.future.whenComplete(action);
+
+  /// Substitutes [newFuture] for the currently wrapped [Future], which is
+  /// returned.
+  Future<T> substitute(Future<T> newFuture) {
+    if (_complete) {
+      throw new StateError("You may not call substitute on a SubstituteFuture "
+          "that's already complete.");
+    }
+
+    var oldFuture = _inner;
+    _inner = newFuture;
+    _inner.then((value) {
+      if (_inner != newFuture) return;
+      _completer.complete(value);
+      _complete = true;
+    }).catchError((error) {
+      if (_inner != newFuture) return;
+      _completer.completeError(error);
+      _complete = true;
+    });
+    return oldFuture;
+  }
+}