code: function shouldTranspileDependency( filepath ) {
  	// find the last index of node_modules and check from there
  	// we want <working>/node_modules/a-package/node_modules/foo/index.js to only match foo, not a-package
  	const marker = '/node_modules/';
  	const lastIndex = filepath.lastIndexOf( marker );
  	if ( lastIndex === -1 ) {
  		// we're not in node_modules
  		return false;
  	}
  
  	const checkFrom = lastIndex + marker.length;
  
  	return _.some(
  		nodeModulesToTranspile,
  		modulePart => filepath.substring( checkFrom, checkFrom + modulePart.length ) === modulePart
  	);
  }
docs: Check to see if we should transpile certain files in node_modules
  @param {String} filepath the path of the file to check
  @returns {Boolean} True if we should transpile it, false if not
  
  We had a thought to try to find the package.json and use the engines property
  to determine what we should transpile, but not all libraries set engines properly
  (see d3-array@2.0.0). Instead, we transpile libraries we know to have dropped Node 4 support
  are likely to remain so going forward.