Fixes https://github.com/flutter/flutter/issues/36091.

Previously, a sendRequest will be sent even if the peer is closed during
a driver test. That will cause a time out without any error information.
Such issue is unreproducible on my Mac Book Pro, but 100% reproducible
on our new Mac mini (2018).

The closing issue is tracked in https://github.com/flutter/flutter/issues/36268

Additional to this fix, we should also patch the jason_rpc_2 so the peer
will throw exception if sendRequest is attempted while the connection is
closed.

**Test**:
tiles_scroll_perf_iphonexs__timeline_summary failed without this patch.
It will pass after this patch.

I'm not sure how to add a unit test for this. Please let me know if you have
some ideas.

This patch will generate the following warning log on the new Mac Mini
```
flutter: Observatory listening on http://127.0.0.1:50192/cZPDF4sW7MM=/  
Installing and launching...                                        10.2s
00:00 +0: scrolling performance test (setUpAll)
[info ] FlutterDriver: Connecting to Flutter application at http://127.0.0.1:1069/cZPDF4sW7MM=/
[trace] FlutterDriver: Isolate found with number: 3684677742843303
[trace] FlutterDriver: Isolate is paused at start.
[trace] FlutterDriver: Attempting to resume isolate
[trace] FlutterDriver: Waiting for service extension
[info ] FlutterDriver: Connected to Flutter application.
00:00 +0: scrolling performance test complex_layout_scroll_perf
[warning] FlutterDriver: Instance of '_WebSocketImpl' is closed with an unexpected code 1005
[warning] FlutterDriver: Peer connection is closed! Trying to restore the connection...
00:10 +1: scrolling performance test tiles_scroll_perf
[warning] FlutterDriver: Instance of '_WebSocketImpl' is closed with an unexpected code 1005
[warning] FlutterDriver: Peer connection is closed! Trying to restore the connection...
00:20 +2: scrolling performance test (tearDownAll)
00:20 +2: All tests passed!
Stopping application instance.
```