[api-minor] Move the ReadableStream polyfill to the global scope

Note that most (reasonably) modern browsers have supported this for a while now, see https://developer.mozilla.org/en-US/docs/Web/API/ReadableStream#Browser_compatibility

By moving the polyfill into `src/shared/compatibility.js` we can thus get rid of the need to manually export/import `ReadableStream` and simply use it directly instead.

The only change here which *could* possibly lead to a difference in behavior is in the `isFetchSupported` function. Previously we attempted to check for the existence of a global `ReadableStream` implementation, which could now pass (assuming obviously that the preceding checks also succeeded).
However I'm not sure if that's a problem, since the previous check only confirmed the existence of a native `ReadableStream` implementation and not that it actually worked correctly. Finally it *could* just as well have been a globally registered polyfill from an application embedding the PDF.js library.

src/display/display_utils.js

@@ -435,7 +435,6 @@ class StatTimer {
 function isFetchSupported() {
   return (typeof fetch !== 'undefined' &&
           typeof Response !== 'undefined' && 'body' in Response.prototype &&
-          // eslint-disable-next-line no-restricted-globals
           typeof ReadableStream !== 'undefined');
 }
 

src/pdf.js

@@ -106,7 +106,6 @@ exports.createObjectURL = pdfjsSharedUtil.createObjectURL;
 exports.removeNullCharacters = pdfjsSharedUtil.removeNullCharacters;
 exports.shadow = pdfjsSharedUtil.shadow;
 exports.Util = pdfjsSharedUtil.Util;
-exports.ReadableStream = pdfjsSharedUtil.ReadableStream;
 exports.RenderingCancelledException =
   pdfjsDisplayDisplayUtils.RenderingCancelledException;
 exports.getFilenameFromUrl = pdfjsDisplayDisplayUtils.getFilenameFromUrl;

src/shared/compatibility.js

@@ -249,6 +249,36 @@ const isIE = /Trident/.test(userAgent);
   globalThis.URL = require('core-js/web/url');
 })();
 
+// Support: IE, Node.js
+(function checkReadableStream() {
+  if (typeof PDFJSDev !== 'undefined' && PDFJSDev.test('IMAGE_DECODERS')) {
+    // The current image decoders are synchronous, hence `ReadableStream`
+    // shouldn't need to be polyfilled for the IMAGE_DECODERS build target.
+    return;
+  }
+  let isReadableStreamSupported = false;
+
+  if (typeof ReadableStream !== 'undefined') {
+    // MS Edge may say it has ReadableStream but they are not up to spec yet.
+    try {
+      // eslint-disable-next-line no-new
+      new ReadableStream({
+        start(controller) {
+          controller.close();
+        },
+      });
+      isReadableStreamSupported = true;
+    } catch (e) {
+      // The ReadableStream constructor cannot be used.
+    }
+  }
+  if (isReadableStreamSupported) {
+    return;
+  }
+  globalThis.ReadableStream =
+    require('web-streams-polyfill/dist/ponyfill').ReadableStream;
+})();
+
 // Support: IE<11, Safari<8, Chrome<36
 (function checkWeakMap() {
   if (globalThis.WeakMap) {

src/shared/message_handler.js

@@ -16,7 +16,7 @@
 
 import {
   AbortException, assert, createPromiseCapability, MissingPDFException,
-  ReadableStream, UnexpectedResponseException, UnknownErrorException
+  UnexpectedResponseException, UnknownErrorException
 } from './util';
 
 const CallbackKind = {

src/shared/streams_polyfill.js

@@ -1,55 +0,0 @@
-/* Copyright 2017 Mozilla Foundation
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-/* eslint-disable no-restricted-globals */
-
-if (typeof PDFJSDev !== 'undefined' && PDFJSDev.test('MOZCENTRAL')) {
-  if (typeof ReadableStream === 'undefined') {
-    throw new Error('Please enable ReadableStream support by resetting the ' +
-      '"javascript.options.streams" preference to "true" in about:config.');
-  }
-  exports.ReadableStream = ReadableStream;
-} else {
-  let isReadableStreamSupported = false;
-  if (typeof ReadableStream !== 'undefined') {
-    // MS Edge may say it has ReadableStream but they are not up to spec yet.
-    try {
-      // eslint-disable-next-line no-new
-      new ReadableStream({
-        start(controller) {
-          controller.close();
-        },
-      });
-      isReadableStreamSupported = true;
-    } catch (e) {
-      // The ReadableStream constructor cannot be used.
-    }
-  }
-  if (isReadableStreamSupported) {
-    exports.ReadableStream = ReadableStream;
-  } else if (typeof PDFJSDev !== 'undefined' &&
-             PDFJSDev.test('IMAGE_DECODERS')) {
-    class DummyReadableStream {
-      constructor() {
-        throw new Error('The current image decoders are synchronous, ' +
-                        'hence `ReadableStream` shouldn\'t need to be ' +
-                        'polyfilled for the IMAGE_DECODERS build target.');
-      }
-    }
-    exports.ReadableStream = DummyReadableStream;
-  } else {
-    exports.ReadableStream =
-      require('web-streams-polyfill/dist/ponyfill').ReadableStream;
-  }
-}

src/shared/util.js

@@ -15,7 +15,6 @@
 /* eslint no-var: error */
 
 import './compatibility';
-import { ReadableStream } from './streams_polyfill';
 
 const IDENTITY_MATRIX = [1, 0, 0, 1, 0, 0];
 const FONT_IDENTITY_MATRIX = [0.001, 0, 0, 0.001, 0, 0];
@@ -930,7 +929,6 @@ export {
   readUint16,
   readUint32,
   removeNullCharacters,
-  ReadableStream,
   setVerbosityLevel,
   shadow,
   string32,