fully support untyped mode

[rkirov]

As per our conversation with JS compiler folks, most advanced optimizations do not need types (only disambiguate/ambiguate props do). Sickle can fully support an advanced optimization consumer once we have the following:

• turn off closure warnings/errors on valid code that is minification safe. switch untyped mode to emit @suppress {checkTypes} instead of @type {?}. #59
• support a driver that does typechecking first, then sickle run, finally an emit (no typechecking).
• detect minification unsafe code that passes the TS typechecker. Warn when properties are accessed through ['literal'] #23

/cc @martine