Avoids adding "elementClasses" twice

Components may add elementClasses themselves for some reason (like progressive enhancement for Soy). In this case we should avoid adding it again, or the classes will show up twice in each element.

packages/metal-incremental-dom/src/IncrementalDomRenderer.js

@@ -48,7 +48,7 @@ class IncrementalDomRenderer extends ComponentRenderer {
 	 */
 	addElementClasses_(elementClasses, args) {
 		for (var i = 3; i < args.length; i += 2) {
-			if (args[i] === 'class') {
+			if (args[i] === 'class' && args[i + 1].indexOf(elementClasses) === -1) {
 				args[i + 1] += ' ' + elementClasses;
 				return;
 			}

packages/metal-incremental-dom/test/IncrementalDomRenderer.js

@@ -425,6 +425,20 @@ describe('IncrementalDomRenderer', function() {
 		});
 	});
 
+	it('should not cause css classes to be added twice due to "elementClasses"', function() {
+		class TestComponent extends Component {
+			render() {
+				IncDom.elementVoid('div', null, null, 'class', this.elementClasses);
+			}
+		}
+		TestComponent.RENDERER = IncrementalDomRenderer;
+
+		component = new TestComponent({
+			elementClasses: 'test'
+		});
+		assert.strictEqual('test', component.element.getAttribute('class'));
+	});
+
 	describe('Existing Content', function() {
 		it('should not change existing content if the same that would be rendered', function() {
 			class TestComponent extends Component {