fix(isDisabled): do not modify item

Previously the item from the list was modified with the
_uiSelectChoiceDisabled property. This allowed a leakage of information
from ui-select to outside the directive. It also caused issues when the
was used outside of the directive.

This commit adds a reference array to store disabled items and so
prevents the need to modify the item in place.

Closes #1200 and #1661

Partially supersedes #1641

src/uiSelectController.js

@@ -324,18 +324,42 @@ uis.controller('uiSelectCtrl',
         ctrl.selected.filter(function (selection) { return angular.equals(selection, item); }).length > 0);
   };
 
+  var disabledItems = [];
+
+  function _updateItemDisabled(item, isDisabled) {
+    var disabledItemIndex = disabledItems.indexOf(item);
+    if (isDisabled && disabledItemIndex === -1) {
+      disabledItems.push(item);
+    }
+
+    if (!isDisabled && disabledItemIndex > -1) {
+      disabledItems.splice(disabledItemIndex, 0);
+    }
+  }
+
+  function _isItemDisabled(item) {
+    return disabledItems.indexOf(item) > -1;
+  }
+
   ctrl.isDisabled = function(itemScope) {
 
     if (!ctrl.open) return;
 
-    var itemIndex = ctrl.items.indexOf(itemScope[ctrl.itemProperty]);
+    var item = itemScope[ctrl.itemProperty];
+    var itemIndex = ctrl.items.indexOf(item);
     var isDisabled = false;
-    var item;
+
+    if (itemIndex >= 0 && (angular.isDefined(ctrl.disableChoiceExpression) || ctrl.multiple)) {
 
-    if (itemIndex >= 0 && (!angular.isUndefined(ctrl.disableChoiceExpression) || ctrl.multiple)) {
-      item = ctrl.items[itemIndex];
-      isDisabled = !!(itemScope.$eval(ctrl.disableChoiceExpression)) || _isItemSelected(item); // force the boolean value
-      item._uiSelectChoiceDisabled = isDisabled; // store this for later reference
+      if (ctrl.multiple) {
+        isDisabled = _isItemSelected(item);
+      }
+
+      if (!isDisabled && angular.isDefined(ctrl.disableChoiceExpression)) {
+        isDisabled = !!(itemScope.$eval(ctrl.disableChoiceExpression));
+      }
+
+      _updateItemDisabled(item, isDisabled);
     }
 
     return isDisabled;
@@ -344,11 +368,11 @@ uis.controller('uiSelectCtrl',
 
   // When the user selects an item with ENTER or clicks the dropdown
   ctrl.select = function(item, skipFocusser, $event) {
-    if (item === undefined || !item._uiSelectChoiceDisabled) {
+    if (item === undefined || !_isItemDisabled(item)) {
 
       if ( ! ctrl.items && ! ctrl.search && ! ctrl.tagging.isActivated) return;
 
-      if (!item || !item._uiSelectChoiceDisabled) {
+      if (!item || !_isItemDisabled(item)) {
         if(ctrl.tagging.isActivated) {
           // if taggingLabel is disabled and item is undefined we pull from ctrl.search
           if ( ctrl.taggingLabel === false ) {