chore(has-alt): use virtualNode instead of node (dequelabs#1923)

* chore(has-alt): use virtualNode instead of node

* chore(has-alt): ignore null alt

lib/checks/shared/has-alt.js

@@ -1,4 +1,6 @@
-var nn = node.nodeName.toLowerCase();
-return (
-	node.hasAttribute('alt') && (nn === 'img' || nn === 'input' || nn === 'area')
-);
+const { nodeName } = virtualNode.props;
+if (!['img', 'input', 'area'].includes(nodeName)) {
+	return false;
+}
+
+return virtualNode.hasAttr('alt');

test/checks/shared/has-alt.js

@@ -2,23 +2,29 @@ describe('has-alt', function() {
 	'use strict';
 
 	var fixture = document.getElementById('fixture');
+	var checkSetup = axe.testUtils.checkSetup;
 
 	afterEach(function() {
 		fixture.innerHTML = '';
 	});
 
 	it('should return true if an alt is present', function() {
-		var node = document.createElement('img');
-		node.setAttribute('alt', 'woohoo');
-		fixture.appendChild(node);
+		var checkArgs = checkSetup('<img id="target" alt="woohoo" />');
+		assert.isTrue(checks['has-alt'].evaluate.apply(null, checkArgs));
+	});
 
-		assert.isTrue(checks['has-alt'].evaluate(node));
+	it('should return true if an empty alt is present', function() {
+		var checkArgs = checkSetup('<img id="target" alt="" />');
+		assert.isTrue(checks['has-alt'].evaluate.apply(null, checkArgs));
 	});
 
-	it('should return false if an alt is not present', function() {
-		var node = document.createElement('img');
-		fixture.appendChild(node);
+	it('should return true if a null alt is present', function() {
+		var checkArgs = checkSetup('<img id="target" alt />');
+		assert.isTrue(checks['has-alt'].evaluate.apply(null, checkArgs));
+	});
 
-		assert.isFalse(checks['has-alt'].evaluate(node));
+	it('should return false if an alt is not present', function() {
+		var checkArgs = checkSetup('<img id="target" />');
+		assert.isFalse(checks['has-alt'].evaluate.apply(null, checkArgs));
 	});
 });