Unhandled exception: Instance of 'MissingPortFile'

```
$ webdev serve
[INFO] Building new asset graph completed, took 1.6s
[INFO] Checking for unexpected pre-existing outputs. completed, took 1ms
[INFO] Unhandled exception:
Instance of 'MissingPortFile'
#0 _existingPort (package:build_daemon/client.dart:24:37)

#1 BuildDaemonClient.connect (package:build_daemon/client.dart:186:15)

#2 connectClient (package:webdev/src/daemon_client.dart:17:23)
#3 _startBuildDaemon (package:webdev/src/serve/dev_workflow.dart:26:18)
#4 DevWorkflow.start (package:webdev/src/serve/dev_workflow.dart:194:24)
#5 ServeCommand.run (package:webdev/src/command/serve_command.dart:101:27)

#6 CommandRunner.runCommand (package:args/command_runner.dart:197:27)
#7 _CommandRunner.runCommand (package:webdev/src/webdev_command_runner.dart:38:24)
#8 CommandRunner.run. (package:args/command_runner.dart:112:25)
#9 new Future.sync (dart:async/future.dart:224:31)
#10 CommandRunner.run (package:args/command_runner.dart:112:14)
#11 run (package:webdev/src/webdev_command_runner.dart:19:56)
#12 main (file:///opt/pub-cache/hosted/pub.dartlang.org/webdev-2.5.4/bin/webdev.dart:17:22)
#13 _startIsolate. (dart:isolate-patch/isolate_patch.dart:305:32)
#14 _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:174:12)
```

hi, I can't serve nor build web-angular with these errors, even after using ```stagehand web-angular```

Reproduce the failure with `stagehand web-angular` followed by `webdev serve`, then start at `_existingPort` in `package:build_daemon/client.dart` and follow the callers in `package:webdev/src/daemon_client.dart` and `package:webdev/src/serve/dev_workflow.dart`. Determine why the port file is missing and define the expected behavior for both `serve` and `build`; done means neither command exits with this unhandled exception.

Written by the indexing model from the issue text.