I'm submitting a...

[x] Other... Please describe: Tracking issue for internal cleanup.

What needs to be done

In order to quickly turn on strictPropertyInitialization flag in the whole code base, we introduced ! on every non-initialized class field. Each one of those fields needs to be individually checked and decided whether:

• the field should be initialized. Thus foo!: string, should become foo = 'default'.
• the field should be marked as optional. Thus foo!: string, should become foo?: string.
• the field is reliably initialized, but TS cannot infer that (initialization can occur outside the ctor, for example), so ! should be kept.