Revert "Add count and countWhere extensions."

[natebosch]

Reverts #196

These cause a conflict internally. Revert for now.

[lrhn]

When would "for now" end?
Do we have a plan to address the conflict?
Should a conflict somewhere be able to block a change that is internally consistent for the package it's added to?

[natebosch]

When would "for now" end?
Do we have a plan to address the conflict?

When we discussed it last I don't think we had a concrete solution or decision about rolling through the breakages. We had decided that this one could be rolled back since there wasn't a strong need for It. When we come to a decision on dart-lang/core#620 we can either decided to never add these, or decide to add them with some rollout plan.

Should a conflict somewhere be able to block a change that is internally consistent for the package it's added to?

I'm not sure. Maybe? Hopefully not though.