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

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

+// 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;
+    }
+  }
+
+  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';
+}