These are only used for type checking, so don't bother importing them at
runtime.
Generally add future annotations at the same time, to make sure that
existing uses of these imports don't need to be quoted.
This is the final refactoring for extracting the bultin object
logic out of Interpreterbase. I decided to do both arrays and
dicts in one go since splitting it would have been a lot more
confusing.
Eli Schwartz