Make isEmpty a getter.

tests/html/htmlcollection_test.dart

 #library('ElementListTest');
 #import('../../pkg/unittest/unittest.dart');
 #import('../../pkg/unittest/html_config.dart');
 #import('dart:html');
 
 // Test that List<Element> implements List<T>
 main() {
   Element insertTestDiv() {
     Element element = new Element.tag('div');
     element.innerHTML = r"""
@@ skipping 112 matching lines @@
 
     List<Element> someChecked =
         document.query('#someChecked').elements;
 
     List<Element> emptyDiv =
         document.query('#emptyDiv').elements;
 
     Expect.equals(4, someChecked.length);
     Expect.equals(0, emptyDiv.length);
 
-    Expect.isFalse(someChecked.isEmpty());
-    Expect.isTrue(emptyDiv.isEmpty());
+    Expect.isFalse(someChecked.isEmpty);
+    Expect.isTrue(emptyDiv.isEmpty);
 
     root.remove();
   });
 
   int countWithForEach(collection, predicate) {
     int count = 0;
     collection.forEach((element) {
         if (predicate(element)) count++;
       });
     return count;
@@ skipping 123 matching lines @@
     Expect.equals(-1, someChecked.lastIndexOf(someChecked[3], 2));
 
     Expect.equals(-1, someChecked.lastIndexOf(noneChecked[0], 3));
     Expect.equals(-1, noneChecked.lastIndexOf(someChecked[0], 3));
     Expect.equals(-1, someChecked.lastIndexOf(noneChecked[1], 3));
     Expect.equals(-1, noneChecked.lastIndexOf(someChecked[1], 3));
 
     root.remove();
   });
 }