Expose Zones.

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 6b6c760dadb760119dcc4140a9ee492121fd5368..03e23e76252497165bf33abe4d4853c9a73298cd 100644
--- a/sdk/lib/async/future.dart
+++ b/sdk/lib/async/future.dart
@@ -194,12 +194,15 @@ abstract class Future<T> {
     // List collecting values from the futures.
     // Set to null if an error occurs.
     List values;
-    void handleError(error) {
+
+    handleError(error) {
       if (values != null) {
         values = null;
         completer.completeError(error);
       }
+      return null;
     }
+
     // As each future completes, put its value into the corresponding
     // position in the list of values.
     int remaining = 0;
@@ -365,7 +368,7 @@ abstract class Future<T> {
  *
  * If you already have a Future, you probably don't need a Completer.
  * Instead, you can usually use [Future.then], which returns a Future:
- * 
+ *
  *     Future doStuff(){
  *       return someAsyncOperation().then((result) {
  *         // Do something.