packages/build-tools/utils/manifest.js
function flattenDeep(arr1) {
  return arr1.reduce(
    (acc, val) =>
      Array.isArray(val) ? acc.concat(flattenDeep(val)) : acc.concat(val),
    [],
  );
}
function flattenDeep(arr1) {
  return arr1.reduce(
    (acc, val) =>
      Array.isArray(val) ? acc.concat(flattenDeep(val)) : acc.concat(val),
    [],
  );
}
function flattenDeep ( arr1 ) { return arr1 . reduce ( ( acc , val ) => Array . isArray ( val ) ? acc . concat ( flattenDeep ( val ) ) : acc . concat ( val ) , [ ] , ) ; }
recursively flatten heavily nested arrays
recursively flatten heavily nested arrays