fix(css): add empty class check to NgElement.

Dart2js 1.10 started passes classNames through without performing empty string
check. DOM apis throw on empty string, so we perform the empty class
name check inside our APIs.

Author: rkirov

lib/core_dom/ng_element.dart

@@ -36,12 +36,14 @@ class NgElement {
 
   /// Schedules a DOM write adding [className] to the element.
   void addClass(String className) {
+    if (className.isEmpty) return;
     _scheduleDomWrite();
     _classesToUpdate[className] = true;
   }
 
   /// Schedules a DOM write removing [className] from the element.
   void removeClass(String className) {
+    if (className.isEmpty) return;
     _scheduleDomWrite();
     _classesToUpdate[className] = false;
   }

test/core_dom/ng_element_spec.dart

@@ -18,7 +18,7 @@ void main() {
         var element = e('<div></div>');
         var ngElement = new NgElement(element, scope, animate);
 
-        ngElement..addClass('one')..addClass('two three');
+        ngElement..addClass('one')..addClass('two')..addClass('three');
 
         ['one', 'two', 'three'].forEach((className) {
           expect(element).not.toHaveClass(className);
@@ -54,6 +54,19 @@ void main() {
         expect(element).toHaveClass('four');
       });
 
+      it('should ignore empty strings',
+          (TestBed _, Animate animate) {
+
+        var scope = _.rootScope;
+        var element = e('<div></div>');
+        var ngElement = new NgElement(element, scope, animate);
+
+        ngElement..addClass('');
+        ngElement..removeClass('');
+
+        scope.apply();
+      });
+
       it('should always apply the last dom operation on the given className',
           (TestBed _, Animate animate) {