asyncio: Make Tasks check if Futures are attached to the same event loop

See https://github.com/python/asyncio/pull/303 for details

Lib/asyncio/tasks.py

@@ -251,7 +251,13 @@ class Task(futures.Future):
         else:
             if isinstance(result, futures.Future):
                 # Yielded Future must come from Future.__iter__().
-                if result._blocking:
+                if result._loop is not self._loop:
+                    self._loop.call_soon(
+                        self._step,
+                        RuntimeError(
+                            'Task {!r} got Future {!r} attached to a '
+                            'different loop'.format(self, result)))
+                elif result._blocking:
                     result._blocking = False
                     result.add_done_callback(self._wakeup)
                     self._fut_waiter = result