fix future onError handler wrapping to avoid cast error

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 5b570c606f6869df24fbe6a621772e8f32634bf5..355cd4c7606113807c63de5e303ed05cb1652294 100644
--- a/sdk/lib/async/future.dart
+++ b/sdk/lib/async/future.dart
@@ -304,7 +304,9 @@ abstract class Future<T> {
     StackTrace stackTrace;  // The stackTrace that came with the error.
 
     // Handle an error from any of the futures.
-    void handleError(theError, theStackTrace) {
+    // TODO(jmesserly): use `void` return type once it can be inferred for the
+    // `then` call below.
+    handleError(theError, theStackTrace) {
       remaining--;
       if (values != null) {
         if (cleanUp != null) {