[PERF] Guard against poorly optimized babel output

[runspired]

This PR will fail to build until we have fixed all the locations in which our babel compilation step is producing sub-optimal output to handle let and const closures.

When this PR is green, we can merge it and gain perfs.

cc @stefanpenner

[stefanpenner]

the thrown error is good (from ci):

The Broccoli Plugin: [broccoli-persistent-filter:Babel] failed with:
SyntaxError: ember-data/serializers/rest.js: Compiling let/const in this block would add a closure (throwIfClosureRequired).
  254 |     let keys = Object.keys(payload);
  255 | 
> 256 |     for (let i = 0, length = keys.length; i < length; i++) {
      |                                                            ^
  257 |       let prop = keys[i];
  258 |       let modelName = prop;
  259 |       let forcedSecondary = false;

[stefanpenner]

@runspired should i fix?

[runspired]

@stefanpenner yes. However, it stops the build on the first error encountered, you may just encounter the next one :)

[stefanpenner]

looking now

[stefanpenner]

this made me finally fix: broccolijs/broccoli-persistent-filter@d7ab2d1

[stefanpenner]

oops broke something...

[stefanpenner]

oops broke something...

fixed locally, lets see what try says.

[runspired]

Thanks @stefanpenner :)