Add missing null-tests to async error functions.

sdk/lib/async/zone.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.
 
 part of dart.async;
 
 typedef dynamic ZoneCallback();
 typedef dynamic ZoneUnaryCallback(arg);
 typedef dynamic ZoneBinaryCallback(arg1, arg2);
 
@@ skipping 18 matching lines @@
     Zone self, ZoneDelegate parent, Zone zone, Duration duration, void f());
 typedef Timer CreatePeriodicTimerHandler(
     Zone self, ZoneDelegate parent, Zone zone,
     Duration period, void f(Timer timer));
 typedef void PrintHandler(
     Zone self, ZoneDelegate parent, Zone zone, String line);
 typedef Zone ForkHandler(Zone self, ZoneDelegate parent, Zone zone,
                          ZoneSpecification specification,
                          Map zoneValues);
 
-/// Pair of error and stack trace. Returned by [Zone.errorCallback].
+/** Pair of error and stack trace. Returned by [Zone.errorCallback]. */
 class AsyncError implements Error {
   final error;
   final StackTrace stackTrace;
 
   AsyncError(this.error, this.stackTrace);
+
   String toString() => error.toString();
 }
 
 
 class _ZoneFunction {
   final _Zone zone;
   final Function function;
   const _ZoneFunction(this.zone, this.function);
 }
 
@@ skipping 192 matching lines @@
 /**
  * A Zone represents the asynchronous version of a dynamic extent. Asynchronous
  * callbacks are executed in the zone they have been queued in. For example,
  * the callback of a `future.then` is executed in the same zone as the one where
  * the `then` was invoked.
  */
 abstract class Zone {
   // Private constructor so that it is not possible instantiate a Zone class.
   Zone._();
 
-  /// The root zone that is implicitly created.
+  /** The root zone that is implicitly created. */
   static const Zone ROOT = _ROOT_ZONE;
 
-  /// The currently running zone.
+  /** The currently running zone. */
   static Zone _current = _ROOT_ZONE;
 
   static Zone get current => _current;
 
   dynamic handleUncaughtError(error, StackTrace stackTrace);
 
   /**
    * Returns the parent zone.
    *
    * Returns `null` if `this` is the [ROOT] zone.
@@ skipping 969 matching lines @@
                                    handleUncaughtError: errorHandler);
   }
   Zone zone = Zone.current.fork(specification: zoneSpecification,
                                 zoneValues: zoneValues);
   if (onError != null) {
     return zone.runGuarded(body);
   } else {
     return zone.run(body);
   }
 }