Error when terminating web connections.

[isaldana]

When calling terminate() on a GRPC web channel the following exception is thrown:

error closing channel: Concurrent modification during iteration: Instance of '_HashSet<XhrTransportStream>'.

Repro steps

1. Create multiple web streams.
2. Call terminate() on the GRPC web channel.

Expected result: I expect existing calls and streams to be terminated.

Actual result: An exception is thrown and it looks like existing calls are kept open. The following exception is also displayed.

Details

Error: Bad state: Cannot add event after closing
    at Object.throw_ [as throw] (http://localhost:49717/dart_sdk.js:4330:11)
    at _AsyncStreamController.new.addError (http://localhost:49717/dart_sdk.js:33436:55)
    at ClientCall.new.[_onRequestError] (http://localhost:49717/packages/grpc/src/client/connection.dart.lib.js:538:26)
    at http://localhost:49717/packages/grpc/src/client/transport/xhr_transport.dart.lib.js:138:27
    at Object._checkAndCall (http://localhost:49717/dart_sdk.js:4540:16)
    at Object.dcall (http://localhost:49717/dart_sdk.js:4545:17)
    at XMLHttpRequest.<anonymous> (http://localhost:49717/dart_sdk.js:104837:21)
    at xhr_transport.XhrTransportStream.new.terminate (http://localhost:49717/packages/grpc/src/client/transport/xhr_transport.dart.lib.js:108:25)
    at terminate.next (<anonymous>)
    at runBody (http://localhost:49717/dart_sdk.js:37387:34)
    at Object._async [as async] (http://localhost:49717/dart_sdk.js:37418:7)
    at xhr_transport.XhrTransportStream.new.terminate (http://localhost:49717/packages/grpc/src/client/transport/xhr_transport.dart.lib.js:106:20)
    at ClientCall.new._terminate (http://localhost:49717/packages/grpc/src/client/connection.dart.lib.js:571:36)
    at _terminate.next (<anonymous>)
    at runBody (http://localhost:49717/dart_sdk.js:37387:34)
    at Object._async [as async] (http://localhost:49717/dart_sdk.js:37418:7)
    at ClientCall.new.[_terminate] (http://localhost:49717/packages/grpc/src/client/connection.dart.lib.js:564:22)
    at ClientCall.new._safeTerminate (http://localhost:49717/packages/grpc/src/client/connection.dart.lib.js:585:35)
    at _safeTerminate.next (<anonymous>)
    at runBody (http://localhost:49717/dart_sdk.js:37387:34)
    at Object._async [as async] (http://localhost:49717/dart_sdk.js:37418:7)
    at ClientCall.new.[_safeTerminate] (http://localhost:49717/packages/grpc/src/client/connection.dart.lib.js:583:22)
    at ClientCall.new.[_onTimedOut] (http://localhost:49717/packages/grpc/src/client/connection.dart.lib.js:442:29)
    at internalCallback (http://localhost:49717/dart_sdk.js:23542:11)