[traverseAllChildren] fix out-of-scope var use.

Dear ES6 gods, bring us `let` soon.

This fixes an issue where non-keyed iterables are used as children and the value of `i` would be undefined because its used out of scope. This adds a separately scoped iteration index value and appropriately increments it as iteration continues. Doi.

While I'm in there, make the usage of falsey `nameSoFar` more obvious and more consistent with the existing usage on L115

Test plan: first wrote a test covering this previously untested path. Saw an identical issue as was experienced in development environment. Then ensured test passed after this diff.
Merge pull request #2458 from leebyron/fix-iterable

[traverseAllChildren] fix out-of-scope var use.