Flag Synchronous option in XMLHttpRequest.open

[erosman]

Example:
https://reviewers.addons.mozilla.org/en-US/firefox/files/compare/1002571...991553/file/bgr.js#L12

httpGet = function(url)
{
    var xmlHttp = new XMLHttpRequest();
    xmlHttp.open( "GET", url, false );
    xmlHttp.send( null );
    return xmlHttp.responseText;
}

[EnTeQuAk]

@erosman Why should we flag this? Is that a rule in our policy somewhere?

I'm sure there are good reasons to flag this, e.g synchronous calls may block user ui etc but generally, it's hard to make this false-positive proof since the synchronous request can be hidden in some async code somewhere.

Not sure what the benefit would be here.

cc @wagnerand

[wagnerand]

@EnTeQuAk I think your example talks about a false negative. I think we can live with that since we'll never reach 100% coverage using those static checks and this warning is to point out obvious mistakes instead of trying to identify sneaky behavior.

P3 is fine though, it's not a high priority.

[stale]

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