Add a ScheduledProcess class to pkg/scheduled_test.

pkg/scheduled_test/lib/src/value_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 value_future;
+
+import 'dart:async';
+
+/// A [Future] wrapper that provides synchronous access to the value of the
+/// wrapped [Future] once it's completed.
+class ValueFuture<T> implements Future<T> {
+  /// The wrapped [Future].
+  Future<T> _future;
+
+  /// The [value] of the wrapped [Future], if it's completed succesfully. If it
+  /// hasn't completed yet or has completed with an error, this will be `null`.
+  T get value => _value;
+  T _value;
+
+  /// Whether the wrapped [Future] has completed successfully.
+  bool get hasValue => _hasValue;
+  var _hasValue = false;
+
+  ValueFuture(Future<T> future) {
+    _future = future.then((value) {
+      _value = value;
+      _hasValue = true;
+      return value;
+    });
+  }
+
+  Stream<T> asStream() => _future.asStream();
+  Future catchError(onError(AsyncError asyncError), {bool test(error)}) =>
+    _future.catchError(onError, test: test);
+  Future then(onValue(T value), {onError(AsyncError asyncError)}) =>
+    _future.then(onValue, onError: onError);
+  Future<T> whenComplete(action()) => _future.whenComplete(action);
+}