Add Iterable.fold (and Stream.fold) which replace `reduce`.

For now this is just a copy. In a next step we will change the behavior of
`reduce`.

BUG= http://dartbug.com/9536

Review URL: https://codereview.chromium.org//13548002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@20978 260f80e4-7a28-3924-810f-c04153c831b5

lib/src/io.dart

@@ -328,7 +328,7 @@ Future<bool> confirm(String message) {
 /// Reads and discards all output from [stream]. Returns a [Future] that
 /// completes when the stream is closed.
 Future drainStream(Stream stream) {
-  return stream.reduce(null, (x, y) {});
+  return stream.fold(null, (x, y) {});
 }
 
 /// Returns a [EventSink] that pipes all data to [consumer] and a [Future] that