feat(ngAria): announce ngMessages with aria-live

By including the `ngAria` module, `ngMessages` will automatically include the aria-live
attribute with an assertive voice, allowing validation messages to be spoken throuhg a
screenreader.

Closes #9834

Author: Marcy Sutton

src/ngAria/aria.js

@@ -238,5 +238,16 @@ ngAriaModule.directive('ngShow', ['$aria', function($aria) {
 .directive('ngDisabled', ['$aria', function($aria) {
   return $aria.$$watchExpr('ngDisabled', 'aria-disabled');
 }])
+.directive('ngMessages', function() {
+  return {
+    restrict: 'A',
+    require: '?ngMessages',
+    link: function(scope, elem, attr, ngMessages) {
+      if (!elem.attr('aria-live')) {
+        elem.attr('aria-live', 'assertive');
+      }
+    }
+  };
+})
 .directive('ngClick', ngAriaTabindex)
 .directive('ngDblclick', ngAriaTabindex);

test/ngAria/ariaSpec.js

@@ -400,6 +400,16 @@ describe('$aria', function() {
     });
   });
 
+  describe('announcing ngMessages', function() {
+    beforeEach(injectScopeAndCompiler);
+
+    it('should attach aria-live', function() {
+      var element = [
+        $compile('<div ng-messages="myForm.myName.$error">')(scope)
+      ];
+      expectAriaAttrOnEachElement(element, 'aria-live', "assertive");
+    });
+  });
 
   describe('aria-value when disabled', function() {
     beforeEach(configAriaProvider({