mock out createObjectURL for makeSchema task

Author: alexcjohnson

tasks/util/make_schema.js

@@ -12,6 +12,7 @@ module.exports = function makeSchema(plotlyPath, schemaPath) {
         // package is annoying and platform-dependent.
         // see https://github.com/tmpvar/jsdom/issues/1782
         w.HTMLCanvasElement.prototype.getContext = function() { return null; };
+        w.URL.createObjectURL = function() { return null; };
 
         w.eval(plotlyjsCode);