polymer - Adding <th>'s to Vaadin Grid Dynamically -

when adding columns dynamically how can set <th>'s in grids header.

 <vaadin-grid id="grid">          <table>           <colgroup>           </colgroup>           <thead>             <tr>               <th>adfdfa</th>               <th>adfdfa</th>             </tr>           </thead>         </table>       </vaadin-grid>     _addcols: function() {     var inputs = this.selectedform.inputs;     var grid = this.$.grid;     grid.columns = [];     for(var = 0; < inputs.length; i++) {       var input = inputs[i];       grid.addcolumn({name: input.$key}, i);      }   }, 

no matter found in docs.

 grid.header.getcell(0, 0).content = 'project name'; 

documentation