internal: lazy load fs due to circular dependency

The fs module requires internal/fs and internal/fs requires fs which
causes a circular dependency. This change lazy loads fs inside of
internal/fs to prevent assertEncoding from breaking when requiring via
stdin.

Fixes: nodejs#11257

lib/internal/fs.js

@@ -2,7 +2,6 @@
 
 const Buffer = require('buffer').Buffer;
 const Writable = require('stream').Writable;
-const fs = require('fs');
 const util = require('util');
 const constants = process.binding('constants').fs;
 
@@ -15,6 +14,15 @@ const O_SYNC = constants.O_SYNC | 0;
 const O_TRUNC = constants.O_TRUNC | 0;
 const O_WRONLY = constants.O_WRONLY | 0;
 
+let fs;
+// Lazy load fs due to a circular dependency.
+function lazyFS() {
+  if (!fs) {
+    fs = require('fs');
+  }
+  return fs;
+}
+
 function assertEncoding(encoding) {
   if (encoding && !Buffer.isEncoding(encoding)) {
     throw new Error(`Unknown encoding: ${encoding}`);
@@ -72,7 +80,7 @@ function SyncWriteStream(fd, options) {
 util.inherits(SyncWriteStream, Writable);
 
 SyncWriteStream.prototype._write = function(chunk, encoding, cb) {
-  fs.writeSync(this.fd, chunk, 0, chunk.length);
+  lazyFS().writeSync(this.fd, chunk, 0, chunk.length);
   cb();
   return true;
 };
@@ -82,7 +90,7 @@ SyncWriteStream.prototype._destroy = function() {
     return;
 
   if (this.autoClose)
-    fs.closeSync(this.fd);
+    lazyFS().closeSync(this.fd);
 
   this.fd = null;
   return true;

test/parallel/test-stdin-require.js

@@ -0,0 +1,20 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const execSync = require('child_process').execSync;
+const fs = require('fs');
+const path = require('path');
+
+common.refreshTmpDir();
+const filename = path.join(__dirname, '..', 'fixtures', 'baz.js');
+const out = path.join(common.tmpDir, 'js.out');
+const bin = process.execPath;
+const input = `require('${filename}'); console.log('PASS');`;
+const cmd = common.isWindows ?
+  `echo "${input}" | ${bin} > ${out} 2>&1; more ${out}` :
+  `echo "${input}" | ${bin} &> ${out}; cat ${out}`;
+
+const result = execSync(cmd).toString();
+assert.strictEqual(result.trim(), 'PASS');
+fs.unlinkSync(out);