source-map-support uses deprecated XMLHttpRequest on main thread

FYI lately, Chrome warns me about something source-map-support does:

"Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience. For more help, check http://xhr.spec.whatwg.org/."

It points to this bit of code:

```
function retrieveSourceMapURL(source) {
var fileData;

if (isInBrowser()) {
var xhr = new XMLHttpRequest();
xhr.open('GET', source, false);
xhr.send(null);
fileData = xhr.readyState === 4 ? xhr.responseText : null;

// Support providing a sourceMappingURL via the SourceMap header
var sourceMapHeader = xhr.getResponseHeader("SourceMap") ||
xhr.getResponseHeader("X-SourceMap");
if (sourceMapHeader) {
return sourceMapHeader;
}
}
```

Maybe this needs to be done differently in the future.