Merge pull request #9201 from ydfzgyj/btoa

Add `btoa` back to domstubs.js

examples/node/domstubs.js

@@ -197,6 +197,10 @@ DOMElementSerializer.prototype = {
   },
 };
 
+function btoa (chars) {
+  return Buffer.from(chars, 'binary').toString('base64');
+}
+
 const document = {
   childNodes : [],
 
@@ -241,6 +245,7 @@ Image.prototype = {
   }
 }
 
+exports.btoa = btoa;
 exports.document = document;
 exports.Image = Image;