Adding polyfills for MouseEvent.offsetX and MouseEvent.offsetY so they work on FF.

lib/html/doc/interface/MouseEvent.dartdoc

Index: lib/html/doc/interface/MouseEvent.dartdoc
diff --git a/lib/html/doc/interface/MouseEvent.dartdoc b/lib/html/doc/interface/MouseEvent.dartdoc
index c4cf4985f76e652d7ca8b09e7ab3897addc0f041..41ec254f10a8f0292b7c593450e3e92506fdf835 100644
--- a/lib/html/doc/interface/MouseEvent.dartdoc
+++ b/lib/html/doc/interface/MouseEvent.dartdoc
@@ -39,10 +39,20 @@ interface MouseEvent extends UIEvent default _MouseEventFactoryProvider {
   /** @domName MouseEvent.metaKey */
   final bool metaKey;
 
-  /** @domName MouseEvent.offsetX */
+  /**
+   * The X coordinate of the mouse pointer in target node coordinates.
+   * This value may vary between platforms if the target node moves
+   * after the event has fired or if the element has CSS transforms affecting
+   * it.
+   */
   final int offsetX;
 
-  /** @domName MouseEvent.offsetY */
+  /**
+   * The Y coordinate of the mouse pointer in target node coordinates.
+   * This value may vary between platforms if the target node moves
+   * after the event has fired or if the element has CSS transforms affecting
+   * it.
+   */
   final int offsetY;
 
   /** @domName MouseEvent.relatedTarget */