fix(select): throw for selectAs and trackBy

trackBy and selectAs have never worked together, and are fundamentally
incompatible since model changes cannot deterministically be
reflected back to the view. This change throws an error to help
developers better understand this scenario.

docs/content/error/ngOptions/trkslct.ngdoc

@@ -0,0 +1,30 @@
+@ngdoc error
+@name ngOptions:trkslct
+@fullName Comprehension expression cannot contain both `select as` and `track by` expressions.
+@description
+
+This error occurs when 'ngOptions' is passed a comprehension expression that contains both a
+`select as` expression and a `track by` expression. These two expressions are fundamentally
+incompatible.
+
+ * Example of bad expression: `<select ng-options="item.subItem as item.label for item in values track by item.id" ng-model="selected">`
+   `values: [{id: 1, label: 'aLabel', subItem: {name: 'aSubItem'}}, {id: 2, label: 'bLabel', subItem: {name: 'bSubItem'}}]`,
+   `$scope.selected = {name: 'aSubItem'};`
+ * track by is always applied to `value`, with purpose to preserve the selection,
+   (to `item` in this case)
+ * To calculate whether an item is selected, `ngOptions` does the following:
+   1. apply `track by` to the values in the array:
+      In the example: [1,2]
+   2. apply `track by` to the already selected value in `ngModel`:
+      In the example: this is not possible, as `track by` refers to `item.id`, but the selected
+      value from `ngModel` is `{name: aSubItem}`.
+
+Here's an example of how to make this example work by using `track by` without `select as`:
+
+```
+<select ng-model="selected" ng-options="item.label for item in values track by item.id">
+```
+
+Note: This would store the whole `item` as the model to `scope.selected` instead of `item.subItem`.
+
+For more information on valid expression syntax, see 'ngOptions' in {@link ng.directive:select select} directive docs.