instanceof narrowing inference not working on references

[idoros]

var htmlElement:HTMLElement = getElement()

// 
var offsetParent = htmlElement.offsetParent;
if(offsetParent instanceof HTMLElement){
    htmlElement = offsetParent; // OK!
}

// alternatively this will cause an error:
if(htmlElement.offsetParent instanceof HTMLElement){
    htmlElement = htmlElement.offsetParent; // ERROR: Assigned expression type Element is not assignable to type HTMLElement 
}

[mhegazy]

looks like a duplicate of #1260