Add a ResetTimer class.

lib/src/restartable_timer.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.restartable_timer;
+
+import 'dart:async';
+
+/// A non-periodic timer that can be restarted any number of times.
+///
+/// Once restarted (via [reset]), the timer counts down from its original
+/// duration again.
+class RestartableTimer implements Timer {
+  /// The duration of the timer.
+  final Duration _duration;
+
+  /// The callback to call when the timer fires.
+  final ZoneCallback _callback;
+
+  /// The timer for the current or most recent countdown.
+  ///
+  /// This timer is canceled and overwritten every time this [RestartableTimer]
+  /// is reset.
+  Timer _timer;
+
+  /// Creates a new timer.
+  ///
+  /// The [callback] function is invoked after the given [duration]. Unlike a
+  /// normal non-periodic [Timer], [callback] may be called more than once.
+  RestartableTimer(this._duration, this._callback) {
+    _timer = new Timer(_duration, _callback);
+  }
+
+  bool get isActive => _timer.isActive;
+
+  /// Restarts the timer so that it counts down from its original duration
+  /// again.
+  ///
+  /// This restarts the timer even if it has already fired or has been canceled.
+  void reset() {
+    _timer.cancel();
+    _timer = new Timer(_duration, _callback);
+  }
+
+  void cancel() {
+    _timer.cancel();
+  }
+}