code: function defineBooleanAttribute (key, handler) {
      var attr = $attrs.$normalize('md-' + key);
      if (handler) defineProperty(key, handler);
      if ($attrs.hasOwnProperty(attr)) updateValue($attrs[attr]);
      $attrs.$observe(attr, updateValue);
      function updateValue (newValue) {
        ctrl[ key ] = newValue !== 'false';
      }
    }
docs: Defines boolean attributes with default value set to true. I.e. md-stretch-tabs with no value
  will be treated as being truthy.
  @param {string} key
  @param {Function} handler