The code in this sample a little old, as I think ItemFileReadStore is deprecated, but I was able to get it to work by removing the "identifier" in your data. I think the "identifier" has to a numeric field... I think.
This worked for me
var query = new esri.tasks.Query();
query.where = "1=1";
query.returnGeometry = false;
query.returnDistinctValues = true;
query.outfields = ["FIELD_NAME"];
//query.orderByFields = ["FIELD_NAME"]; // only works in 10.1 services, this one is 10.01
getList.queryFeatures(query, function (featureSet) {
//Populate dropdown list
var values = dojo.map(featureSet.features, function (feature) {
return {
name: feature.attributes.FIELD_NAME
};
});
var dataItems = {
label: 'name',
items: values
};
var store = new dojo.data.ItemFileReadStore({
data: dataItems
});
...
That populated the dropdown with the field names.
Here is the docs on Combox with samples using the dojo/store to populate the dropdown.