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>

Author: BridgeAR

lib/_stream_readable.js

@@ -38,11 +38,11 @@ const {
 ERR_METHOD_NOT_IMPLEMENTED,
 ERR_STREAM_UNSHIFT_AFTER_END_EVENT
 }=require('internal/errors').codes;
-constReadableAsyncIterator=require('internal/streams/async_iterator');
 const{ emitExperimentalWarning }=require('internal/util');
 
 // Lazy loaded to improve the startup performance.
 letStringDecoder;
+letReadableAsyncIterator;
 
 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');
 returnnewReadableAsyncIterator(this);
 };