Dart Debug Extension doesn't work with DWDS example

[elliette]

Steps to reproduce:

• Follow the instructions in CONTRIBUTING to start the example app
• After opening Dart DevTools, close it and instead click the Dart Debug Extension

Expected: Dart DevTools opens
Actual: Dart DevTools never opens

If you go to chrome://extensions, toggle Dev mode on, and click the link for the background script in the Dart Debug Extension, the following error is shown:

Error handling response: Bad state: Cannot add event after closing

[annagrin]

Figured this out:) You will need to pass --debug-extension flag to the webdev serve... command.
We should either enable this setting by default or add an alert to the user in the case it is not enabled.