Switch to using observer from event listeners.

This lets us to track when page is unloaded.
Implement simple `when` helper for events.
Add interface to get a promise for next state.
Add simple test for dev/panel.
Add missing dev/utils module.
Fix observer leak in the frame script.
Fix typo in the when implementation.
Add a communication test.
Make sure that only local icon's are supported.
Fix typo in the debuggee close method.
Enable back panel validation.
Add test to check a debugger client.