title: dynamic select with relation to other selection
body: <p>I have two methods to get data from remote service.</p>
  
  <p>Api.types() returns me [{name:'vegetable', id:0}, {name:'fruit', id:1}]. Based on that returns and if i change selected type it should update <strong>variety</strong> field data with data about fruits/vegetables. What's wrong at that code?</p>
  
  <pre><code>controller('MainCtrl', ['$log', 'Api', function($log, Api) {
    this.formFields = [
      {
         key: "type",
         type: "select",
         templateOptions: {
           label: "Types",
           valueProp: "id",
           labelProp: "name",
           options:[],
         },
         expressionProperties: {
           'templateOptions.options': function($viewValue, $modelValue, scope) {
              Api.types()
                .then(function(ok) { scope.to.options=ok;}, function(error){});
           }
         },
      },
      {
         key: "variety",
         type: "select",
         templateOptions: {
           label: "variety",
           valueProp: "id",
           labelProp: "name",
           options:[],
         },
         expressionProperties: {
           'templateOptions.options': function($viewValue, $modelValue, scope) {
              Api.variety(scope.model.type)
               .then(function(ok) { scope.to.options=ok;}, function(error){});
           }
         },
      }
    ];
  
  });
  </code></pre>