babel error with Set() and spread syntax, when using "loose":true

.babelrc

{
  "presets": [["env", {
      "loose": true
    }
  ]]
}

package.json

{
  "name": "test-babel-set-spread",
  "version": "1.0.0",
  "description": "",
  "scripts": {
    "test:node": "node test-set-spread.js",
    "test:babel": "babel test-set-spread.js | node",
    "build:babel": "babel test-set-spread.js -o test-set-spread.babel.js"
  },
  "license": "ISC",
  "dependencies": {
    "babel-cli": "^6.26.0",
    "babel-preset-env": "^1.6.1"
  }
}

test-set-spread.babel.js

'use strict';

var priorSet = new Set(['a', 'b']);
console.log('prior Set:', priorSet);

var withSpread = new Set([].concat(priorSet, ['e']));
console.log('with spread syntax:', withSpread);

var withArrayFrom = new Set(Array.from(priorSet).concat(['e']));
console.log('with Array.from:', withArrayFrom);

test-set-spread.js

const priorSet = new Set(['a','b']);
console.log('prior Set:', priorSet);

const withSpread = new Set([...priorSet, 'e']);
console.log('with spread syntax:', withSpread);

const withArrayFrom = new Set(Array.from(priorSet).concat(['e']));
console.log('with Array.from:', withArrayFrom);

tests-output.md

$: npm install

$: npm run test:node

prior Set: Set { 'a', 'b' }
with spread syntax: Set { 'a', 'b', 'e' }
with Array.from: Set { 'a', 'b', 'e' }

$: npm run test:babel

prior Set: Set { 'a', 'b' }
with spread syntax: Set { Set { 'a', 'b' }, 'e' }
with Array.from: Set { 'a', 'b', 'e' }

Demonstrates a syntax issue when compiling code using babel-env and setting loose option to true.

test-rest-spread.js - original syntax
test-rest-spread.babel.js - when compiled with babel

The demonstrations above (npm run test:node / npm run test:babel) illustrate that the output is not equivalent when running the original code or the code produced by babel.

This line is the issue; it works normally within node, and creates a new Set containing the prior Set and an added value:

const withSpread = new Set([...priorSet, 'e']);

But when processed via babel and loose:true, it becomes this:

var withSpread = new Set([].concat(priorSet, ['e']));

Which ends up creating a new Set() containing the prior one as an element, instead of spreading its contents.

The final line of the code shows an alternative syntax which will pass through babel unharmed, and will work on either end:

const withArrayFrom = new Set(Array.from(priorSet).concat(['e']));