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chromium/third_party/blink/web_tests/http/tests/xmlhttprequest/send-object-tostring-check.html 

<!DOCTYPE html>
<html>
<head>
<script src="/js-test-resources/js-test.js"></script>
<script>
description('Test to verify XMLHttpRequest.send(...), with an object, calls toString exactly once.');
var testObj = new function() {
    this.callCounter = 0;
    this.toString = function() {
        this.callCounter++;
        return 'Hello World';
    };
}

xhr = new XMLHttpRequest;
xhr.open('GET', 'resources/get.txt', true);
xhr.send(testObj);
shouldBe('testObj.callCounter', '1');
</script>
</head>
</html>