Add missing null-tests to async error functions.

sdk/lib/async/future.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;
 
 /**
  * An object representing a delayed computation.
  *
  * A [Future] is used to represent a potential value, or error,
@@ skipping 169 matching lines @@
    *
    * Use [Completer] to create a Future and complete it later.
    */
   factory Future.value([value]) {
     return new _Future<T>.immediate(value);
   }
 
   /**
    * A future that completes with an error in the next event-loop iteration.
    *
-   * Use [Completer] to create a Future and complete it later.
+   * If [error] is `null`, it is replaced by a [NullThrownError].
+   *
+   * Use [Completer] to create a future and complete it later.
    */
   factory Future.error(Object error, [StackTrace stackTrace]) {
+    error = _nonNullError(error);
     if (!identical(Zone.current, _ROOT_ZONE)) {
       AsyncError replacement = Zone.current.errorCallback(error, stackTrace);
       if (replacement != null) {
-        error = replacement.error;
+        error = _nonNullError(replacement.error);
         stackTrace = replacement.stackTrace;
       }
     }
     return new _Future<T>.immediateError(error, stackTrace);
   }
 
   /**
    * Creates a future that runs its computation after a delay.
    *
    * The [computation] will be executed after the given [duration] has passed,
@@ skipping 449 matching lines @@
   /**
    * Whether the future has been completed.
    */
   bool get isCompleted;
 }
 
 // Helper function completing a _Future with error, but checking the zone
 // for error replacement first.
 void _completeWithErrorCallback(_Future result, error, stackTrace) {
   AsyncError replacement = Zone.current.errorCallback(error, stackTrace);
-  if (replacement == null) {
-    result._completeError(error, stackTrace);
-  } else {
-    result._completeError(replacement.error, replacement.stackTrace);
+  if (replacement != null) {
+    error = _nonNullError(replacement.error);
+    stackTrace = replacement.stackTrace;
   }
+  result._completeError(error, stackTrace);
 }
 
+/** Helper function that converts `null` to a [NullThrownError]. */
+Object _nonNullError(Object error) =>
+  (error != null) ? error : new NullThrownError();