diff --git a/lib/fs.js b/lib/fs.js
index 6e28a41118e131..a965c84afc1369 100644
--- a/lib/fs.js
+++ b/lib/fs.js
@@ -16,6 +16,7 @@ const FSReqWrap = binding.FSReqWrap;
 const FSEvent = process.binding('fs_event_wrap').FSEvent;
 const internalFS = require('internal/fs');
 const assertEncoding = internalFS.assertEncoding;
+const stringToFlags = internalFS.stringToFlags;
 const SyncWriteStream = internalFS.SyncWriteStream;
 
 Object.defineProperty(exports, 'constants', {
@@ -30,15 +31,6 @@ const Writable = Stream.Writable;
 const kMinPoolSpace = 128;
 const kMaxLength = require('buffer').kMaxLength;
 
-const O_APPEND = constants.O_APPEND || 0;
-const O_CREAT = constants.O_CREAT || 0;
-const O_EXCL = constants.O_EXCL || 0;
-const O_RDONLY = constants.O_RDONLY || 0;
-const O_RDWR = constants.O_RDWR || 0;
-const O_SYNC = constants.O_SYNC || 0;
-const O_TRUNC = constants.O_TRUNC || 0;
-const O_WRONLY = constants.O_WRONLY || 0;
-
 const isWindows = process.platform === 'win32';
 
 const DEBUG = process.env.NODE_DEBUG && /fs/.test(process.env.NODE_DEBUG);
@@ -522,48 +514,6 @@ fs.readFileSync = function(path, options) {
 };
 
 
-// Used by binding.open and friends
-function stringToFlags(flag) {
-  // Return early if it's a number
-  if (typeof flag === 'number') {
-    return flag;
-  }
-
-  switch (flag) {
-    case 'r' : return O_RDONLY;
-    case 'rs' : // fall through
-    case 'sr' : return O_RDONLY | O_SYNC;
-    case 'r+' : return O_RDWR;
-    case 'rs+' : // fall through
-    case 'sr+' : return O_RDWR | O_SYNC;
-
-    case 'w' : return O_TRUNC | O_CREAT | O_WRONLY;
-    case 'wx' : // fall through
-    case 'xw' : return O_TRUNC | O_CREAT | O_WRONLY | O_EXCL;
-
-    case 'w+' : return O_TRUNC | O_CREAT | O_RDWR;
-    case 'wx+': // fall through
-    case 'xw+': return O_TRUNC | O_CREAT | O_RDWR | O_EXCL;
-
-    case 'a' : return O_APPEND | O_CREAT | O_WRONLY;
-    case 'ax' : // fall through
-    case 'xa' : return O_APPEND | O_CREAT | O_WRONLY | O_EXCL;
-
-    case 'a+' : return O_APPEND | O_CREAT | O_RDWR;
-    case 'ax+': // fall through
-    case 'xa+': return O_APPEND | O_CREAT | O_RDWR | O_EXCL;
-  }
-
-  throw new Error('Unknown file open flag: ' + flag);
-}
-
-// exported but hidden, only used by test/simple/test-fs-open-flags.js
-Object.defineProperty(exports, '_stringToFlags', {
-  enumerable: false,
-  value: stringToFlags
-});
-
-
 // Yes, the follow could be easily DRYed up but I provide the explicit
 // list to make the arguments clear.
 
diff --git a/lib/internal/fs.js b/lib/internal/fs.js
index dcfb4be1236530..41b4905f1d5bb8 100644
--- a/lib/internal/fs.js
+++ b/lib/internal/fs.js
@@ -4,6 +4,16 @@ const Buffer = require('buffer').Buffer;
 const Stream = require('stream').Stream;
 const fs = require('fs');
 const util = require('util');
+const constants = process.binding('constants').fs;
+
+const O_APPEND = constants.O_APPEND | 0;
+const O_CREAT = constants.O_CREAT | 0;
+const O_EXCL = constants.O_EXCL | 0;
+const O_RDONLY = constants.O_RDONLY | 0;
+const O_RDWR = constants.O_RDWR | 0;
+const O_SYNC = constants.O_SYNC | 0;
+const O_TRUNC = constants.O_TRUNC | 0;
+const O_WRONLY = constants.O_WRONLY | 0;
 
 function assertEncoding(encoding) {
   if (encoding && !Buffer.isEncoding(encoding)) {
@@ -12,6 +22,40 @@ function assertEncoding(encoding) {
 }
 exports.assertEncoding = assertEncoding;
 
+function stringToFlags(flag) {
+  if (typeof flag === 'number') {
+    return flag;
+  }
+
+  switch (flag) {
+    case 'r' : return O_RDONLY;
+    case 'rs' : // Fall through.
+    case 'sr' : return O_RDONLY | O_SYNC;
+    case 'r+' : return O_RDWR;
+    case 'rs+' : // Fall through.
+    case 'sr+' : return O_RDWR | O_SYNC;
+
+    case 'w' : return O_TRUNC | O_CREAT | O_WRONLY;
+    case 'wx' : // Fall through.
+    case 'xw' : return O_TRUNC | O_CREAT | O_WRONLY | O_EXCL;
+
+    case 'w+' : return O_TRUNC | O_CREAT | O_RDWR;
+    case 'wx+': // Fall through.
+    case 'xw+': return O_TRUNC | O_CREAT | O_RDWR | O_EXCL;
+
+    case 'a' : return O_APPEND | O_CREAT | O_WRONLY;
+    case 'ax' : // Fall through.
+    case 'xa' : return O_APPEND | O_CREAT | O_WRONLY | O_EXCL;
+
+    case 'a+' : return O_APPEND | O_CREAT | O_RDWR;
+    case 'ax+': // Fall through.
+    case 'xa+': return O_APPEND | O_CREAT | O_RDWR | O_EXCL;
+  }
+
+  throw new Error('Unknown file open flag: ' + flag);
+}
+exports.stringToFlags = stringToFlags;
+
 // Temporary hack for process.stdout and process.stderr when piped to files.
 function SyncWriteStream(fd, options) {
   Stream.call(this);
diff --git a/test/parallel/test-fs-open-flags.js b/test/parallel/test-fs-open-flags.js
index d212130bc903d1..ce7eaad580c008 100644
--- a/test/parallel/test-fs-open-flags.js
+++ b/test/parallel/test-fs-open-flags.js
@@ -1,3 +1,4 @@
+// Flags: --expose_internals
 'use strict';
 require('../common');
 var assert = require('assert');
@@ -12,39 +13,41 @@ var O_RDWR = fs.constants.O_RDWR || 0;
 var O_TRUNC = fs.constants.O_TRUNC || 0;
 var O_WRONLY = fs.constants.O_WRONLY || 0;
 
-assert.equal(fs._stringToFlags('r'), O_RDONLY);
-assert.equal(fs._stringToFlags('r+'), O_RDWR);
-assert.equal(fs._stringToFlags('w'), O_TRUNC | O_CREAT | O_WRONLY);
-assert.equal(fs._stringToFlags('w+'), O_TRUNC | O_CREAT | O_RDWR);
-assert.equal(fs._stringToFlags('a'), O_APPEND | O_CREAT | O_WRONLY);
-assert.equal(fs._stringToFlags('a+'), O_APPEND | O_CREAT | O_RDWR);
-
-assert.equal(fs._stringToFlags('wx'), O_TRUNC | O_CREAT | O_WRONLY | O_EXCL);
-assert.equal(fs._stringToFlags('xw'), O_TRUNC | O_CREAT | O_WRONLY | O_EXCL);
-assert.equal(fs._stringToFlags('wx+'), O_TRUNC | O_CREAT | O_RDWR | O_EXCL);
-assert.equal(fs._stringToFlags('xw+'), O_TRUNC | O_CREAT | O_RDWR | O_EXCL);
-assert.equal(fs._stringToFlags('ax'), O_APPEND | O_CREAT | O_WRONLY | O_EXCL);
-assert.equal(fs._stringToFlags('xa'), O_APPEND | O_CREAT | O_WRONLY | O_EXCL);
-assert.equal(fs._stringToFlags('ax+'), O_APPEND | O_CREAT | O_RDWR | O_EXCL);
-assert.equal(fs._stringToFlags('xa+'), O_APPEND | O_CREAT | O_RDWR | O_EXCL);
+const { stringToFlags } = require('internal/fs');
+
+assert.equal(stringToFlags('r'), O_RDONLY);
+assert.equal(stringToFlags('r+'), O_RDWR);
+assert.equal(stringToFlags('w'), O_TRUNC | O_CREAT | O_WRONLY);
+assert.equal(stringToFlags('w+'), O_TRUNC | O_CREAT | O_RDWR);
+assert.equal(stringToFlags('a'), O_APPEND | O_CREAT | O_WRONLY);
+assert.equal(stringToFlags('a+'), O_APPEND | O_CREAT | O_RDWR);
+
+assert.equal(stringToFlags('wx'), O_TRUNC | O_CREAT | O_WRONLY | O_EXCL);
+assert.equal(stringToFlags('xw'), O_TRUNC | O_CREAT | O_WRONLY | O_EXCL);
+assert.equal(stringToFlags('wx+'), O_TRUNC | O_CREAT | O_RDWR | O_EXCL);
+assert.equal(stringToFlags('xw+'), O_TRUNC | O_CREAT | O_RDWR | O_EXCL);
+assert.equal(stringToFlags('ax'), O_APPEND | O_CREAT | O_WRONLY | O_EXCL);
+assert.equal(stringToFlags('xa'), O_APPEND | O_CREAT | O_WRONLY | O_EXCL);
+assert.equal(stringToFlags('ax+'), O_APPEND | O_CREAT | O_RDWR | O_EXCL);
+assert.equal(stringToFlags('xa+'), O_APPEND | O_CREAT | O_RDWR | O_EXCL);
 
 ('+ +a +r +w rw wa war raw r++ a++ w++ x +x x+ rx rx+ wxx wax xwx xxx')
   .split(' ')
   .forEach(function(flags) {
-    assert.throws(function() { fs._stringToFlags(flags); });
+    assert.throws(function() { stringToFlags(flags); });
   });
 
 assert.throws(
-  () => fs._stringToFlags({}),
+  () => stringToFlags({}),
   /Unknown file open flag: \[object Object\]/
 );
 
 assert.throws(
-  () => fs._stringToFlags(true),
+  () => stringToFlags(true),
   /Unknown file open flag: true/
 );
 
 assert.throws(
-  () => fs._stringToFlags(null),
+  () => stringToFlags(null),
   /Unknown file open flag: null/
 );