How to suppress variable type within value attribute using ng-options?

Obviously there was a change in how the ngOptions directive is handled. This change is briefly explained in the migration notes for AngularJS 1.4. A more detailed description of the changes can be found in the commit message:

When using ngOptions: the directive applies a surrogate key as the
value of the <option> element. This commit changes the actual string
used as the surrogate key. We now store a string that is computed by
calling hashKey on the item in the options collection; previously it
was the index or key of the item in the collection.

(This is in keeping with the way that the unknown option value is
represented in the select directive.)

Before you might have seen:

<select ng-model="x" ng-option="i in items">
<option value="1">a</option>
<option value="2">b</option>
<option value="3">c</option>
<option value="4">d</option>
</select>

Now it will be something like:

<select ng-model="x" ng-option="i in items">
<option value="string:a">a</option>
<option value="string:b">b</option>
<option value="string:c">c</option>
<option value="string:d">d</option>
</select>

If your application code relied on this value, which it shouldn’t,
then you will need to modify your application to accommodate this. You
may find that you can use the track by feaure of ngOptions as this
provides the ability to specify the key that is stored.

This means that you now need to use track by to get the same result as before. To fix the example in the question it needs to look like this then:

<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.4.0-rc.2/angular.js">
</script>
<script>
  angular.module("selectOptionsTest", []).
    controller("SelectOptionsController", ["$scope", function($scope) {
      $scope.options = [
        {id: 1, label: "Item 1"},
        {id: 2, label: "Item 2"},
        {id: 3, label: "Item 3"}
      ];
    }]);
</script>
<div ng-app="selectOptionsTest" ng-controller="SelectOptionsController">
  <select ng-model="opt" ng-options="option.id as option.label for option in options track by option.id">
  </select>
</div>