Update dart:async to match the Dart repo.

tool/input_sdk/lib/async/timer.dart

 // Copyright (c) 2012, 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.
 
 part of dart.async;
 
 /**
  * A count-down timer that can be configured to fire once or repeatedly.
  *
  * The timer counts down from the specified duration to 0.
@@ skipping 58 matching lines @@
    * when the previous callback started, or when the previous callback was
    * scheduled for - even if the actual callback was delayed.
    */
   factory Timer.periodic(Duration duration,
                          void callback(Timer timer)) {
     if (Zone.current == Zone.ROOT) {
       // No need to bind the callback. We know that the root's timer will
       // be invoked in the root zone.
       return Zone.current.createPeriodicTimer(duration, callback);
     }
-    return Zone.current.createPeriodicTimer(
-        duration, Zone.current.bindUnaryCallback(callback, runGuarded: true));
+    // TODO(floitsch): the return type should be 'void', and the type
+    // should be inferred.
+    var boundCallback = Zone.current.bindUnaryCallback/*<dynamic, Timer>*/(
+        callback, runGuarded: true);
+    return Zone.current.createPeriodicTimer(duration, boundCallback);
   }
 
   /**
    * Runs the given [callback] asynchronously as soon as possible.
    *
    * This function is equivalent to `new Timer(Duration.ZERO, callback)`.
    */
   static void run(void callback()) {
     new Timer(Duration.ZERO, callback);
   }
@@ skipping 11 matching lines @@
    *
    * A periodic timer is active if it has not been canceled.
    */
   bool get isActive;
 
   external static Timer _createTimer(Duration duration, void callback());
   external static Timer _createPeriodicTimer(Duration duration,
                                              void callback(Timer timer));
 }