function getTernFunctionType(fileInfo, offset) {
        postMessage({
            type: MessageIds.TERN_CALLED_FUNC_TYPE_MSG,
            fileInfo: fileInfo,
            offset: offset
        });

        return addPendingRequest(fileInfo.name, offset, MessageIds.TERN_CALLED_FUNC_TYPE_MSG);
    }
Get a Promise for the function type from TernJS.
@param {{type: string, name: string, offsetLines: number, text: string}} fileInfo
- type of update, name of file, and the text of the update.
For "full" updates, the whole text of the file is present. For "part" updates,
the changed portion of the text. For "empty" updates, the file has not been modified
and the text is empty.
@param {{line:number, ch:number}} offset - the line, column info for what we want the function type of.
@return {jQuery.Promise} - a promise that will resolve to the function type of the function being called.