Downlevel emit for let/const inside loops

[ivogabe]

I've implemented a downlevel emit for block scoped variables that are used inside callbacks in a loop.
Example:

for (let i = 0; i < 9; i++) {
  setTimeout(i, () => console.log(i));
}

Is emitted as

var _a = function(i)  {
    setTimeout(i, function () { return console.log(i); });
};
for (var i = 0; i < 9; i++) _a(i);

Todo:

• Formatting, a do ... while() loop is emitted ugly at the moment.
• Downlevel emit of a for of loop.
• Variables declared using destructuring in the head of a for loop aren't added to the closure yet.
• Tests
• Comments

I'm not sure whether the code in checker.ts is ok, should the variable blockScopedBindingInLoop be turned into a NodeFlag?

[vladima]

Some key aspects are still missing in this PR:

• handling of control transfer statements in loops: return, break and continue (both labeled and non-labeled)
• usage of arguments in loops
• function scoped declarations defined in loops, i.e. var

Implementing these items properly is a non-trivial task though certainly doable. When we've originally started the work on downlevel support for let / const we had a design proposal that covered all these aspects. However even with that we still decided to exclude support for let/const that are declared inside loops and captured in closures. Main reason is: converting loop bodies into functions keeps proper semantic for let/const however it also changes runtime behavior and potentially introduces subtle perf/memory issues that will be difficult to discover, i.e. here

Saying that + the fact that we don't have lots of requests for this feature I'm going to close this PR for now. However if later this feature will have high demand we can always reconsider our decision