Consider implementing Annex B.3.3 block scoping and keep behavior in mind

Kind of filing this as a general reminder. Annex B.3.3 extends ES6's rules for block-scoped declarations, for web compatibility. A few examples can be found in https://github.com/babel/babel/issues/5262

While not strictly needed, Babel's transforms are increasingly being run on all kinds of JS files and running the block scoping transform on code that was written to use B.3.3 behavior can in some cases cause the code to become corrupted. For most cases, we can simply suggest not running the transform on this code.

One of my concerns, which motivated filing this, is that we keep this in mind for Babili. We should be sure that it is not at risk of corrupting any scopes that would with in Annex B.3.3-implementing environment.

No file, test, or entry point is identified. Start by reviewing Annex B.3.3 and the examples in Babel issue 5262, then examine the block-scoping transform and Babili's handling of affected scopes. Done means a clear compatibility decision and verified behavior for code relying on Annex B.3.3.

Written by the indexing model from the issue text.