Added offset document measurement to Element.

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 5f54dcc4b2fd96cc0decd7ae404558a5b713f686..56ce509dd63b2312e9e0f47092b28dfe4d6c66a2 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -8675,6 +8675,13 @@ abstract class Element extends Node implements ElementTraversal native "Element"
     TemplateElement.decorate(this);
   }
 
+  /** Provides the coordinates of the element relative to the document. */
+  Point get documentOffset() {

[blois, 2013/07/12 23:10:08]

Seems like it would be more useful to expose something like:
Point offsetTo(Element)

Thoughts?

+    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"); }