promise.js

  showDashboardLink: Ember.computed(
    function() {
      if (this.get('session.isAuthenticated')) {
        return this.get('store').findAll('datasource').then((datasources) => {
          this.get('store').filter('datasource', {}, function(datasource) {
            return datasource.get('id') != null;
          }).then((datasources) => {
            if (datasources.get('length') != 0) {
              return true
            } else {
              return false
            }
          });
        });
      } else {
        return false
      }
    }
  ),

showDashboardLink: false,
watchIsAuthenticated: Ember.observer('session.isAuthenticated', function() {
 if (this.get('session.isAuthenticated')) {
        return this.get('store').findAll('datasource').then((datasources) => {
          this.get('store').filter('datasource', {}, function(datasource) {
            return datasource.get('id') != null;
          }).then(function(datasources) {
            this.set('showDashboardLink', datasources.get('length') !== 0); 
          }.bind(this));
        });
      } else {
         this.set('showDashboardLink', false);
      }
}),

  watchIsAuthenticated: Ember.observer('session.isAuthenticated', () => {
   if (this.get('session.isAuthenticated')) {
          return this.get('store').findAll('datasource').then((datasources) => {
            this.get('store').filter('datasource', {}, (datasource) => {
              return true;
            }).then((datasources) => {
              this.set('showDashboardLink', true);
            }.bind(this));
          });
        } else {
           this.set('showDashboardLink', true);
        }
  }),

(datasources) => {
              this.set('showDashboardLink', true);
            }.bind(this)

unnecesary bind