Fixing WheelEvent.deltaMode polyfill after IDL update.

tools/dom/templates/html/impl/impl_WheelEvent.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.
 
 part of $LIBRARYNAME;
 
 $(ANNOTATIONS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 
   factory WheelEvent(String type,
       {Window view, int deltaX: 0, int deltaY: 0,
@@ skipping 116 matching lines @@
           return detail * 40;
         }
         return detail;
       }
       return 0;
     }
     throw new UnsupportedError(
         '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);
   num get _wheelDeltaX => JS('num', '#.wheelDeltaX', this);
   num get _detail => JS('num', '#.detail', this);
 
   bool get _hasInitMouseScrollEvent =>
       JS('bool', '!!(#.initMouseScrollEvent)', this);
 
   @JSName('initMouseScrollEvent')
@@ skipping 34 matching lines @@
       int deltaX,
       int deltaY,
       int deltaZ,
       int deltaMode) native;
 
 $else
   @DomName('WheelEvent.deltaX')
   num get deltaX => $dom_wheelDeltaX;
   @DomName('WheelEvent.deltaY')
   num get deltaY => $dom_wheelDeltaY;
-
 $endif
 }