[RN] Use fetch instead of XHR

react/features/base/util/loadScript.native.js

 /**
- * Loads a script from a specific source. React Native cannot load a JS
+ * Loads a script from a specific URL. React Native cannot load a JS
  * file/resource/URL via a <script> HTML element, so the implementation
  * fetches the specified src as plain text (e.g. via XMLHttpRequest) and then
  * evaluates the fetched string as JavaScript code (i.e. via the {@link eval}
  * function).
  *
- * @param {string} src - The source from the which the script is to be
- * (down)loaded. Only absolute URLs are supported.
- * @param {Object} options - Additional options.
- * @param {boolean} options.async=true - True to asynchronously load the script
- * or false to synchronously load the script.
+ * @param {string} url - The absolute URL from the which the script is to be
+ * (down)loaded.
  * @returns {void}
  */
-export function loadScript(
-    src,
-    options = {
-        async: true
-    }) {
-    return new Promise((resolve, reject) => {
-        // XXX We are using XMLHttpRequest instead of Fetch API only in order
-        // to be able to do 'sync' requests. If this not needed, this can be
-        // replaced with much simpler and readable fetch().
-        const xhr = new XMLHttpRequest();
+export function loadScript(url) {
+    let fetch;
+    const method = 'GET';
 
-        xhr.open('GET', src, options.async);
-        xhr.responseType = 'text';
+    // Prefer the Fetch API. Apart from the fact that we're fetching the
+    // specified script as a static resource, the Fetch API provides more
+    // detailed errors.
+    if (typeof (fetch = window.fetch) === 'function') {
+        fetch = fetch(url, { method });
+    } else {
+        // Otherwise, fall back to the XMLHttpRequest API.
+        fetch
+            = new Promise(resolve => {
+                const xhr = new XMLHttpRequest();
 
-        xhr.onload = () => {
-            if (xhr.readyState === 4) {
-                if (xhr.status === 200) {
-                    try {
-                        // eslint-disable-next-line no-eval
-                        eval.call(window, xhr.responseText);
-                        resolve();
-                    } catch (e) {
-                        reject(e);
+                xhr.responseType = 'text';
+
+                xhr.onreadystatechange = () => {
+                    if (xhr.readyState === 4) {
+                        resolve(xhr);
                     }
-                } else {
-                    reject(xhr.statusText);
-                }
-            }
-        };
+                };
+
+                xhr.open(method, url, /* async */ true);
+                xhr.send();
+            });
+    }
 
-        xhr.onerror = () => reject(xhr.statusText);
+    return (
+        fetch
+            .then(response => {
+                switch (response.status) {
+                case 200:
+                    return response.responseText || response.text();
+
+                default:
+                    throw response.statusText;
+                }
 
-        xhr.send();
-    });
+            })
+            .then(responseText => {
+                eval.call(window, responseText); // eslint-disable-line no-eval
+            }));
 }