some errors can't be handled in IOWebSocketChannel.connect

Copied from https://github.com/flutter/flutter/issues/21076

`IOWebSocketChannel.connect` suffers from the problem described here:

https://www.dartlang.org/guides/libraries/futures-error-handling#potential-problem-failing-to-register-error-handlers-early

When I use `connect` with a URL that can't be resolved (e.g. by having my wifi turned off), then I get an exception that can't be caught by `channel.stream.handleError(onError)`.

All exceptions should be handleable by `stream.handleError`, particularly as this is what the docs promise: "If there's an error connecting, the channel's stream emits a WebSocketChannelException wrapping that error and then closes."

ref: https://docs.flutter.io/flutter/web_socket_channel.io/IOWebSocketChannel/IOWebSocketChannel.connect.html

```
flutter: #0 new IOWebSocketChannel._withoutSocket. (package:web_socket_channel/io.dart:83:24)
dart-lang/http#1604 _invokeErrorHandler (dart:async/async_error.dart:13:29)
dart-lang/web_socket_channel#2 _HandleErrorStream._handleError (dart:async/stream_pipe.dart:286:9)
dart-lang/web_socket_channel#3 _ForwardingStreamSubscription._handleError (dart:async/stream_pipe.dart:168:13)
dart-lang/http#1605 _rootRunBinary (dart:async/zone.dart:1144:38)
dart-lang/web_socket_channel#5 _CustomZone.runBinary (dart:async/zone.dart:1037:19)
dart-lang/web_socket_channel#6 _CustomZone.runBinaryGuarded (dart:async/zone.dart:939:7)
dart-lang/web_socket_channel#7 _BufferingStreamSubscription._sendError.sendError (dart:async/stream_impl.dart:355:15)
dart-lang/web_socket_channel#8 _BufferingStreamSubscription._sendError (dart:async/stream_impl.dart:373:16)
dart-lang/web_socket_channel#9 _BufferingStreamSubscription._addError (dart:async/stream_impl.dart:272:7)
dart-lang/web_socket_channel#10 _SyncStreamController._sendError (dart:async/stream_controller.dart:767:19)
dart-lang/web_socket_channel#11 _StreamController._addError (dart:async/stream_controller.dart:647:7)
dart-lang/web_socket_channel#12 _rootRunBinary (dart:async/zone.dart:1144:38)
```

```
flutter doctor -v
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```