Remove deprecated flatten dependency

For: brave/brave-browser#5490

index.js

@@ -3,7 +3,6 @@ const BlockStream = require('block-stream2')
 const calcPieceLength = require('piece-length')
 const corePath = require('path')
 const FileReadStream = require('filestream/read')
-const flatten = require('flatten')
 const fs = require('fs')
 const isFile = require('is-file')
 const junk = require('junk')
@@ -13,6 +12,16 @@ const parallel = require('run-parallel')
 const sha1 = require('simple-sha1')
 const stream = require('readable-stream')
 
+// TODO: When Node 10 support is dropped, replace this Array.prototype.flat
+function flat (arr1) {
+  return arr1.reduce(
+    (acc, val) => Array.isArray(val)
+      ? acc.concat(flat(val))
+      : acc.concat(val),
+    []
+  )
+}
+
 const announceList = [
   ['udp://tracker.leechers-paradise.org:6969'],
   ['udp://tracker.coppersurfer.tk:6969'],
@@ -192,7 +201,7 @@ function _parseInput (input, opts, cb) {
       cb(null, file)
     }), (err, files) => {
       if (err) return cb(err)
-      files = flatten(files)
+      files = flat(files)
       cb(null, files, isSingleFileTorrent)
     })
   }
@@ -202,7 +211,7 @@ function getFiles (path, keepRoot, cb) {
   traversePath(path, getFileInfo, (err, files) => {
     if (err) return cb(err)
 
-    if (Array.isArray(files)) files = flatten(files)
+    if (Array.isArray(files)) files = flat(files)
     else files = [files]
 
     path = corePath.normalize(path)

package.json

@@ -17,7 +17,6 @@
     "bencode": "^2.0.0",
     "block-stream2": "^2.0.0",
     "filestream": "^5.0.0",
-    "flatten": "^1.0.2",
     "is-file": "^1.0.0",
     "junk": "^3.1.0",
     "minimist": "^1.1.0",