switch untyped mode to emit @suppress {checkTypes} instead of @type {?}.

[rkirov]

This means a lot less modifications of source and less opportunities to create invalid TS code though that.

[evmar]

I think I concluded that when you mix untyped code with typed code, you need the untyped-ness to propagate. However, I'd like to try doing this one more time before I just close this bug, so I can include an example of why it fails.

[mprobst]

@martine I think John explicit said suppressing type warnings is preferable to {?}, in particular for optimizations to still work and not horribly fail. If just putting @suppress on files breaks other parts in horrible ways, then we of course shouldn't do it, but it's IMHO worth looking into. If only as a stepping stone on the way to completer type coverage.

[evmar]

We now emit types and @Suppress {checkTypes}.