Added offset document measurement to Element.

sdk/lib/html/dart2js/html_dart2js.dart

 /// The Dart HTML library.
 ///
 /// For examples, see
 /// [Dart HTML5 Samples](https://github.com/dart-lang/dart-html5-samples)
 /// on Github.
 library dart.dom.html;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:_collection-dev' hide Symbol, deprecated;
@@ skipping 8657 matching lines @@
   @Experimental()
   bool get isTemplate => tagName == 'TEMPLATE' || _isAttributeTemplate;
 
   void _ensureTemplate() {
     if (!isTemplate) {
       throw new UnsupportedError('$this is not a template.');
     }
     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"); }
 
   @DomName('Element.abortEvent')
   @DocsEditable()
   static const EventStreamProvider<Event> abortEvent = const EventStreamProvider<Event>('abort');
 
   @DomName('Element.beforecopyEvent')
   @DocsEditable()
   static const EventStreamProvider<Event> beforeCopyEvent = const EventStreamProvider<Event>('beforecopy');
@@ skipping 20769 matching lines @@
       _position = nextPosition;
       return true;
     }
     _current = null;
     _position = _array.length;
     return false;
   }
 
   T get current => _current;
 }