Assert that node can have measure function <==> node is a leaf node

Summary: Instead of silently ignorning non-leaf nodes with measure functions, we should assert that we don't create those kinds of trees.

Reviewed By: emilsjolander

Differential Revision: D4130770

fbshipit-source-id: a3ef10a2e63bbc12b5aa07977e4b84c8d59e3ffe