asyncio: sync with Tulip

* Document why set_result() calls are safe
* Cleanup gather(). Use public methods instead of hacks to consume the
  exception of a future.
* sock_connect(): pass directly the fd to _sock_connect_done instead of the
  socket.

Lib/asyncio/selector_events.py

@@ -363,15 +363,15 @@ class BaseSelectorEventLoop(base_events.BaseEventLoop):
                     break
         except BlockingIOError:
             fut.add_done_callback(functools.partial(self._sock_connect_done,
-                                                    sock))
+                                                    fd))
             self.add_writer(fd, self._sock_connect_cb, fut, sock, address)
         except Exception as exc:
             fut.set_exception(exc)
         else:
             fut.set_result(None)
 
-    def _sock_connect_done(self, sock, fut):
-        self.remove_writer(sock.fileno())
+    def _sock_connect_done(self, fd, fut):
+        self.remove_writer(fd)
 
     def _sock_connect_cb(self, fut, sock, address):
         if fut.cancelled():