How do I find out how many items are in my world?

I would like to find out how many hoppers I have in the world. I've tried

/testforblock hopper
/testforblock minecraft:hopper
/testfor hopper

But I get an

Unknown command

You can't do something for the entire world, because to load it all, you would need many times the power of the biggest supercomputer in the word.
There is also no command to do something for all blocks in an area.

If you play in the world alone, you can look at your statistic for how many hoppers you have placed and subtract the number of hoppers you have broken, that should get you pretty close to the number. It does not include hoppers that were broken by explosions.