Update dart:async to match the Dart repo.

tool/input_sdk/lib/async/timer.dart

Index: tool/input_sdk/lib/async/timer.dart
diff --git a/tool/input_sdk/lib/async/timer.dart b/tool/input_sdk/lib/async/timer.dart
index fafe65228db5060519b0498e950edbd4347e6fe2..1bbb65b93307a81837011043cd1f72c014010bdd 100644
--- a/tool/input_sdk/lib/async/timer.dart
+++ b/tool/input_sdk/lib/async/timer.dart
@@ -76,8 +76,11 @@ abstract class Timer {
       // 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);
   }
 
   /**