rollup_bundle/tsc: add --downlevelIteration
rollup_bundle/tsc: add --downlevelIteration

This allows usage of for..of on iterators, even when targeting ES5.

bazelbuild/rules_nodejs,bazelbuild/rules_nodejs,alexeagle/rules_nodejs,alexeagle/rules_nodejs,alexeagle/rules_nodejs,bazelbuild/rules_nodejs,bazelbuild/rules_nodejs,bazelbuild/rules_nodejs,alexeagle/rules_nodejs