Add missing null-tests to async error functions.

sdk/lib/async/future.dart

Index: sdk/lib/async/future.dart
diff --git a/sdk/lib/async/future.dart b/sdk/lib/async/future.dart
index a4971be56e435eadfe74a19b681376117065893b..179748d7c512219056faee595ac86bf3a954559a 100644
--- a/sdk/lib/async/future.dart
+++ b/sdk/lib/async/future.dart
@@ -187,13 +187,17 @@ abstract class Future<T> {
   /**
    * A future that completes with an error in the next event-loop iteration.
    *
-   * Use [Completer] to create a Future and complete it later.
+   * The [error] must not be `null`.
+   *
+   * Use [Completer] to create a future and complete it later.
    */
   factory Future.error(Object error, [StackTrace stackTrace]) {
+    if (error == null) throw new ArgumentError("Error must not be null");

[floitsch, 2014/09/24 12:59:14]

Or make it an asynchronous "NullThrownError" ?

     if (!identical(Zone.current, _ROOT_ZONE)) {
       AsyncError replacement = Zone.current.errorCallback(error, stackTrace);
       if (replacement != null) {
         error = replacement.error;
+        if (error == null) error = new NullThrownError();
         stackTrace = replacement.stackTrace;
       }
     }
@@ -663,10 +667,11 @@ abstract class Completer<T> {
 // 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 = replacement.error;
+    if (error == null) error = new NullThrownError();
+    stackTrace = replacement.stackTrace;
   }
+  result._completeError(error, stackTrace);
 }