asyncio: Sync with github

Lib/asyncio/streams.py

@@ -494,6 +494,9 @@ class StreamReader:
 
     @coroutine
     def readexactly(self, n):
+        if n < 0:
+            raise ValueError('readexactly size can not be less than zero')
+
         if self._exception is not None:
             raise self._exception
 

Lib/test/test_asyncio/test_streams.py

@@ -351,8 +351,8 @@ class StreamReaderTests(test_utils.TestCase):
         self.assertEqual(b'', data)
         self.assertEqual(self.DATA, stream._buffer)
 
-        data = self.loop.run_until_complete(stream.readexactly(-1))
-        self.assertEqual(b'', data)
+        with self.assertRaisesRegexp(ValueError, 'less than zero'):
+            self.loop.run_until_complete(stream.readexactly(-1))
         self.assertEqual(self.DATA, stream._buffer)
 
     def test_readexactly(self):