Fixing WheelEvent.deltaMode polyfill after IDL update.

tools/dom/templates/html/impl/impl_WheelEvent.darttemplate

Index: tools/dom/templates/html/impl/impl_WheelEvent.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_WheelEvent.darttemplate b/tools/dom/templates/html/impl/impl_WheelEvent.darttemplate
index 1ce11692f5e9be0576c67d7fd5125b38da2ed3d8..c49d8f6b20151666a60e45e2472c7dc6c1aca5be 100644
--- a/tools/dom/templates/html/impl/impl_WheelEvent.darttemplate
+++ b/tools/dom/templates/html/impl/impl_WheelEvent.darttemplate
@@ -134,6 +134,15 @@ $if DART2JS
         'deltaX is not supported');
   }
 
+  @DomName('WheelEvent.deltaMode')
+  int get deltaMode {
+    if (JS('bool', '!!(#.deltaMode)', this)) {
+      return JS('int', '#.deltaMode', this);
+    }
+    // If not available then we're poly-filling and doing pixel scroll.
+    return 0;
+  }
+
   num get _deltaY => JS('num', '#.deltaY', this);
   num get _deltaX => JS('num', '#.deltaX', this);
   num get _wheelDelta => JS('num', '#.wheelDelta', this);
@@ -188,6 +197,5 @@ $else
   num get deltaX => $dom_wheelDeltaX;
   @DomName('WheelEvent.deltaY')
   num get deltaY => $dom_wheelDeltaY;
-
 $endif
 }