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

lib/dom/templates/html/dart2js/impl_MouseEvent.darttemplate

Index: lib/dom/templates/html/dart2js/impl_MouseEvent.darttemplate
diff --git a/lib/dom/templates/html/dart2js/impl_MouseEvent.darttemplate b/lib/dom/templates/html/dart2js/impl_MouseEvent.darttemplate
new file mode 100644
index 0000000000000000000000000000000000000000..c414fcd5c4790380baaa3d57b7e66619a479f6b2
--- /dev/null
+++ b/lib/dom/templates/html/dart2js/impl_MouseEvent.darttemplate
@@ -0,0 +1,38 @@
+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
+$!MEMBERS
+
+  int get offsetX() {
+    if (JS('bool', '!!this.offsetX')) {
+      return this._offsetX;
+    } else {
+      // Firefox does not support offsetX.
+      var target = this.target;
+      if (!(target is Element)) {
+        throw const UnsupportedOperationException(
+            'offsetX is only supported on elements');
+      }
+      return this.clientX - this.target.$dom_getBoundingClientRect().left;

[Jacob, 2012/08/27 19:18:51]

perhaps cache this value as an expando? would be odd if on firefox offsetX
changed as you moved an element but on WebKit it didn't.
Other notes: is this calculation right? If an element has a large border or uses
WebKit transforms (e.g. Rotate3D) won't this give an odd result?
I'd suggest adding a comment to the interface warning about the limitations of
using offsetX.

[blois, 2012/08/27 20:12:16]

Caching only really helps if it's set in the constructor, but doing so would
involve unwanted performance overhead. Rather, I'm adding documentation noting
the potential platform limitations.

On 2012/08/27 19:18:51, Jacob wrote:

+    }
+  }
+
+  int get offsetY() {
+    if (JS('bool', '!!this.offsetY')) {
+      return this._offsetY;
+    } else {
+      // Firefox does not support offsetY.
+      var target = this.target;
+      if (!(target is Element)) {
+        throw const UnsupportedOperationException(
+            'offsetX is only supported on elements');
+      }
+      return this.clientY - this.target.$dom_getBoundingClientRect().top;
+    }
+  }
+
+  int get _offsetX() native 'return this.offsetX';
+  int get _offsetY() native 'return this.offsetY';
+}