fix(ngOptions): throw if ngModel is not present

Closes #7047
Closes #12840

BREAKING CHANGE:
`ngOptions` will now throw if `ngModel` is not present on the `select`
element. Previously, having no `ngModel` let `ngOptions` silently
fail, which could lead to hard to debug errors. The change should
therefore not affect any applications, as it simply makes the
requirement more strict and alerts the developer explicitly.

Author: Narretz

src/ng/directive/ngOptions.js

@@ -395,14 +395,11 @@ var ngOptionsDirective = ['$compile', '$parse', function($compile, $parse) {
   return {
     restrict: 'A',
     terminal: true,
-    require: ['select', '?ngModel'],
+    require: ['select', 'ngModel'],
     link: function(scope, selectElement, attr, ctrls) {
 
-      // if ngModel is not defined, we don't need to do anything
-      var ngModelCtrl = ctrls[1];
-      if (!ngModelCtrl) return;
-
       var selectCtrl = ctrls[0];
+      var ngModelCtrl = ctrls[1];
       var multiple = attr.multiple;
 
       // The emptyOption allows the application developer to provide their own custom "empty"

test/ng/directive/ngOptionsSpec.js

@@ -156,10 +156,10 @@ describe('ngOptions', function() {
   });
 
 
-  it('should have optional dependency on ngModel', function() {
+  it('should have a dependency on ngModel', function() {
     expect(function() {
       compile('<select ng-options="item in items"></select>');
-    }).not.toThrow();
+    }).toThrow();
   });