I would be more comfortable if "started = 1" was also executed in the 
NEWLINE case. I'm not sure to understand the exact use of its value.

And do you think if it's possible to add the type==SEMICOLON case?
This would properly show the warning for:
    import sys; as = 3    # on the same line

There are still some far-fetched constructs that do not warn, like:
    import sys as as
but they are probably not worth the trouble.