ignore requires in dead branches

[Rich-Harris]

Via rollup/rollup#855 (comment):

if (process.env.NODE_ENV !== 'production') {
  var ReactElementValidator = require('./ReactElementValidator');
  createElement = ReactElementValidator.createElement;
  createFactory = ReactElementValidator.createFactory;
  cloneElement = ReactElementValidator.cloneElement;
}

If process.env.NODE_ENV has been rewritten as 'production' (i.e. this plugin follows rollup-plugin-replace), we should disregard the require('./ReactElementValidator').