stream: lazy load ReadableAsyncIterator

PR-URL: #20567
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

lib/_stream_readable.js

@@ -38,11 +38,11 @@ const {
   ERR_METHOD_NOT_IMPLEMENTED,
   ERR_STREAM_UNSHIFT_AFTER_END_EVENT
 } = require('internal/errors').codes;
-const ReadableAsyncIterator = require('internal/streams/async_iterator');
 const { emitExperimentalWarning } = require('internal/util');
 
 // Lazy loaded to improve the startup performance.
 let StringDecoder;
+let ReadableAsyncIterator;
 
 util.inherits(Readable, Stream);
 
@@ -988,7 +988,8 @@ Readable.prototype.wrap = function(stream) {
 
 Readable.prototype[Symbol.asyncIterator] = function() {
   emitExperimentalWarning('Readable[Symbol.asyncIterator]');
-
+  if (ReadableAsyncIterator === undefined)
+    ReadableAsyncIterator = require('internal/streams/async_iterator');
   return new ReadableAsyncIterator(this);
 };