Added offset document measurement to Element.

tests/html/element_test.dart

Index: tests/html/element_test.dart
diff --git a/tests/html/element_test.dart b/tests/html/element_test.dart
index 353733501166435b43a97792b189c1d0a43994aa..7e137419a1317a5691d198ce6897c32a5e5872c4 100644
--- a/tests/html/element_test.dart
+++ b/tests/html/element_test.dart
@@ -59,7 +59,7 @@ main() {
       final container = new Element.tag("div");
       container.style.position = 'absolute';
       container.style.top = '8px';
-      container.style.left = '8px';
+      container.style.left = '9px';
       final element = new Element.tag("div");
       element.style.width = '200px';
       element.style.height = '200px';
@@ -72,8 +72,11 @@ main() {
       expect(element.offset.height, greaterThan(100));
       expect(element.scrollWidth, greaterThan(100));
       expect(element.scrollHeight, greaterThan(100));
-      expect(element.getBoundingClientRect().left, 8);
+      expect(element.getBoundingClientRect().left, 9);
       expect(element.getBoundingClientRect().top, 8);
+
+      expect(element.documentOffset.x, 9);
+      expect(element.documentOffset.y, 8);
       container.remove();
     });
   });