http2 server: a response body larger than the peer's window stalls when sent with endStream #1998

On the server side, stream.sendData(body, endStream: true) with a body
larger than the client's stream window (65535 bytes by default) delivers
exactly the first 65535 bytes and then stops; the client's WINDOW_UPDATEs
for that stream have no effect.

Reproduction: a ServerTransportConnection over a plain TCP socket answers
a GET with sendHeaders([Header.ascii(':status', '200')]) then
sendData(Uint8List(200000), endStream: true); a
ClientTransportConnection reads the response. The client receives 65535
bytes and sends WINDOW_UPDATE frames for stream 1 (visible on the wire), and
nothing more arrives. When the body is written in chunks through
outgoingMessages.addStream(...) and the stream is closed afterwards, all
200000 bytes arrive.

It looks as if queuing the end of the stream closes it on this side while
data still waits for flow-control window, so later WINDOW_UPDATEs for it are
no longer applied.