Bug: Webdev duplicates app connections for each refresh.

[schultek]

The webdev daemon does not properly handle app reconnections. For each page refresh a new app connection is setup while the existing ones are still active. This causes duplicate app connections which quickly pile up after a few refreshes.

The code that causes this:

https://github.com/dart-lang/webdev/blob/697f2f7f56517b0678c6256e0834778905acfc0d/webdev/lib/src/daemon/app_domain.dart#L67

dwds.connectedApps will emit a new appConnection whenever the browser refreshes, even if this app was already handled in webdev. The app will have the same id as an already active app in _appStates[appId], but this is never checked. Therefore all event handlers will be duplicated.

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This can be observed simply by printing some logs on the client and refreshing the page:

Steps to reproduce:

1. dart create bugtest -t web
2. cd bugtest
3. webdev daemon (Or launch a dart web debug session from vscode, which uses the daemon)
4. Add print("Hello Web"); inside the main() in web/main.dart
5. Go to chrome (should open automatically) and refresh the page multiple times.

Expected:

After each refresh ONE log event is printed to the terminal, e.g. [{"event":"app.log","params":{"appId":"CxpfkG6Rz+5OTXWmy8kREA==","log":"Hello Web\n"}}]

Actual:

After each refresh DUPLICATE log events are printed in the number of past refreshes. E.g. after the 10th refresh, 10 duplicate log events are printed.