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..e765ee24b06464c34273d8a55d17948f047e30df 100644
--- a/sdk/lib/async/future.dart
+++ b/sdk/lib/async/future.dart
@@ -304,7 +304,7 @@ 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) {
+    handleError(theError, theStackTrace) {

[floitsch, 2017/02/28 19:04:35]

This looks like a temporary problem.

The 'then' in line 335, gets a 'void' function as argument. I'm not sure what
strong mode infers for the function literal, but it must be either `T->Null` or
`T->dynamic`. It should be `T->void`, in which case the `handleError` function
would have the correct type.

I think we should keep a TODO here, to add the `void` back, once `void` is
correctly inferred.

[Jennifer Messerly, 2017/02/28 19:23:31]

sounds great! I'll add that TODO, does this look good?

TODO(jmesserly): use `void` return type once it can be inferred for the `then`
call below.

       remaining--;
       if (values != null) {
         if (cleanUp != null) {