function getJumptoDef(fileInfo, offset) {
        postMessage({
            type: MessageIds.TERN_JUMPTODEF_MSG,
            fileInfo: fileInfo,
            offset: offset
        });

        return addPendingRequest(fileInfo.name, offset, MessageIds.TERN_JUMPTODEF_MSG);
    }
Get a Promise for the definition from TernJS, for the file & offset passed in.
@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 offset in the file the hints should be calculate at
@return {jQuery.Promise} - a promise that will resolve to definition when
     it is done