yarn build:js-dev is broken due to wrong order of arguments

Problem/Motivation

$ npm run build:js-dev

> Drupal@ build:js-dev /Users/dawehner/www/d8/core
> cross-env NODE_ENV=development node BABEL_ENV=legacy ./scripts/js/babel-es6-build.js

internal/modules/cjs/loader.js:573
    throw err;
    ^

Error: Cannot find module '/Users/dawehner/www/d8/core/BABEL_ENV=legacy'
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:571:15)
    at Function.Module._load (internal/modules/cjs/loader.js:497:25)
    at Function.Module.runMain (internal/modules/cjs/loader.js:719:10)
    at startup (internal/bootstrap/node.js:228:19)
    at bootstrapNodeJSCore (internal/bootstrap/node.js:575:3)
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! Drupal@ build:js-dev: `cross-env NODE_ENV=development node BABEL_ENV=legacy ./scripts/js/babel-es6-build.js`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the Drupal@ build:js-dev script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/dawehner/.npm/_logs/2018-10-01T20_22_28_568Z-debug.log

Proposed resolution

Let's reorder the arguments

Remaining tasks

User interface changes

API changes

Data model changes