fix future onError handler wrapping to avoid cast error

sdk/lib/async/future.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart.async;
 
 /// A type representing values that are either `Future<T>` or `T`.
 ///
 /// This class declaration is a public stand-in for an internal
 /// future-or-value generic type. References to this class are resolved to the
@@ skipping 286 matching lines @@
   static Future<List<T>> wait<T>(Iterable<Future<T>> futures,
                            {bool eagerError: false,
                             void cleanUp(T successValue)}) {
     final _Future<List<T>> result = new _Future<List<T>>();
     List<T> values;  // Collects the values. Set to null on error.
     int remaining = 0;  // How many futures are we waiting for.
     var error;   // The first error from a future.
     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) {
           for (var value in values) {
             if (value != null) {
               // Ensure errors from cleanUp are uncaught.
               new Future.sync(() { cleanUp(value); });
             }
           }
         }
@@ skipping 502 matching lines @@
   if (replacement != null) {
     error = _nonNullError(replacement.error);
     stackTrace = replacement.stackTrace;
   }
   result._completeError(error, stackTrace);
 }
 
 /** Helper function that converts `null` to a [NullThrownError]. */
 Object _nonNullError(Object error) =>
   (error != null) ? error : new NullThrownError();