download-svg.js

var svg = document.querySelector('.blocklySvg');

//get svg source.
var serializer = new XMLSerializer();
var source = serializer.serializeToString(svg);

//add name spaces.
if(!source.match(/^<svg[^>]+xmlns="http\:\/\/www\.w3\.org\/2000\/svg"/)){
    source = source.replace(/^<svg/, '<svg xmlns="http://www.w3.org/2000/svg"');
}
if(!source.match(/^<svg[^>]+"http\:\/\/www\.w3\.org\/1999\/xlink"/)){
    source = source.replace(/^<svg/, '<svg xmlns:xlink="http://www.w3.org/1999/xlink"');
}

//add xml declaration
source = '<?xml version="1.0" standalone="no"?>\r\n' + source;

//convert svg source to URI data scheme.
var url = "data:image/svg+xml;charset=utf-8,"+encodeURIComponent(source);

//set url value to a element's href attribute.
var downloadLink = document.createElement("a");
downloadLink.href = url;
downloadLink.download = "newesttree.svg";
document.body.appendChild(downloadLink);
downloadLink.click();
document.body.removeChild(downloadLink);