Added offset document measurement to Element.

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index fbe12b7ebf314a721ad55c8fcbc70f4d5087a426..fac0395e5f3d11aad293fc21f4278318fa419696 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -8990,6 +8990,13 @@ abstract class Element extends Node implements ElementTraversal {
     TemplateElement.decorate(this);
   }
 
+  /** Provides the coordinates of the element relative to the document. */
+  Point get documentOffset() {
+    final docElem = document.documentElement;
+    final box = getBoundingClientRect();
+    return new Point(box.left + window.pageXOffset - docElem.clientLeft,
+                     box.top + window.pageYOffset - docElem.clientTop);
+  }
   // To suppress missing implicit constructor warnings.
   factory Element._() { throw new UnsupportedError("Not supported"); }