Add missing null-tests to async error functions.

sdk/lib/async/future_impl.dart

Index: sdk/lib/async/future_impl.dart
diff --git a/sdk/lib/async/future_impl.dart b/sdk/lib/async/future_impl.dart
index 124b7ab86069554514b84491e5fb8ae53a9bb80e..5ce7b7fab3faf280930afa7c1390db65d3d29599 100644
--- a/sdk/lib/async/future_impl.dart
+++ b/sdk/lib/async/future_impl.dart
@@ -22,6 +22,7 @@ abstract class _Completer<T> implements Completer<T> {
     AsyncError replacement = Zone.current.errorCallback(error, stackTrace);
     if (replacement != null) {
       error = replacement.error;
+      if (error == null) error = new NullThrownError();
       stackTrace = replacement.stackTrace;
     }
     _completeError(error, stackTrace);
@@ -47,7 +48,6 @@ class _AsyncCompleter<T> extends _Completer<T> {
 }
 
 class _SyncCompleter<T> extends _Completer<T> {
-
   void complete([value]) {
     if (!future._mayComplete) throw new StateError("Future already completed");
     future._complete(value);