fix(fetch): always lazy load stream/web

Author: ronag

lib/fetch/body.js

@@ -6,13 +6,18 @@ const { kState } = require('./symbols')
 const { Blob } = require('buffer')
 const { Readable } = require('stream')
 const { NotSupportedError } = require('../core/errors')
-const { ReadableStream } = require('stream/web')
 const { kBodyUsed } = require('../core/symbols')
 const assert = require('assert')
 const nodeUtil = require('util')
 
+let ReadableStream
+
 // https://fetch.spec.whatwg.org/#concept-bodyinit-extract
 function extractBody (object, keepalive = false) {
+  if (!ReadableStream) {
+    ReadableStream = require('stream/web').ReadableStream
+  }
+
   // 1. Let stream be object if object is a ReadableStream object.
   // Otherwise, let stream be a new ReadableStream, and set up stream.
   let stream = object
@@ -161,6 +166,10 @@ function extractBody (object, keepalive = false) {
 
 // https://fetch.spec.whatwg.org/#bodyinit-safely-extract
 function safelyExtractBody (object, keepalive = false) {
+  if (!ReadableStream) {
+    ReadableStream = require('stream/web').ReadableStream
+  }
+
   // To safely extract a body and a `Content-Type` value from
   // a byte sequence or BodyInit object object, run these steps: