JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap-theme.min.css">
<div ng-app="app">
    <div class="container">
        <div bootstrap-columns>
          <div>Column1</div>
          <div>Column2</div>  
        </div>

        <div bootstrap-columns>
          <div>Column1</div>
          <div>Column2</div>  
          <div>Column3</div>  
        </div>
        
       <div bootstrap-columns>
          <div>Column1</div>
          <div>Column2</div>  
          <div></div>  
        </div>
       <div bootstrap-columns>
          <div>Column1</div>
          <div>Column2</div>  
          <div>Column3</div>  
          <div>Column4</div>  
          <div>Column5</div>  
          <div>Column6</div>  
        </div>

    </div>
</div>

CSS

.row div {
  border: solid 1px black;
}

JavaScript

var app = angular.module('app',[]);
app.directive('bootstrapColumns', function() {
    return {
        restrict: 'A',
        link: function(scope, element, attr) {
            if (!element.hasClass('row'))
                element.addClass('row');


            scope.$watch(function() {
                var elems = element[0].childNodes;
                var count =0;
                angular.forEach(elems, function(e) {
               
                    if (e.tagName == 'DIV' && angular.element(e).text() != '')
                        ++count;
                });
                return count;
            },
            function(cols) {
                var colNum = 12 / cols;
                var cssClass = 'col-xs-' + colNum;
                var elems = element[0].childNodes;
                angular.forEach(elems, function(e) {
                    if (e.tagName == 'DIV') {
                        var div = angular.element(e);                  
                        if (div.text() != '' && !div.hasClass(cssClass))
                            div.addClass(cssClass);
                        }
                });
            });
        }
    }
});