Dumping out the Webpack config in production mode, I have this piece of configuration:

"uglifyOptions": {
  "output": {
    "ascii_only": true
  },
  "ie8": false,
  "ecma": 8,
  "warnings": false,
  "mangle": {
    "safari10": true
  },
  "compress": {
    "warnings": false,
    "comparisons": false
  }
}

This seems to be from a5e76b8 and thus introduced in 3.2.1.

This affects us because we are using the remarkable package, which contains some JS like {Afr:"𝔄",}, which Babel properly converts to {Afr:"\uD835\uDD04",}, but then Uglify shortens back to {Afr:"\u{1d504}",}. This latter syntax isn't supported by IE11, causing issues in production, specifically the error "Expected hexadecimal digit".

Related to #1211