record that Gamepad.buttons creates GamepadButtons

tools/dom/templates/immutable_list_mixin.darttemplate

   // -- start List<$E> mixins.
   // $E is the element type.
 
 $if DEFINE_LENGTH_AS_NUM_ITEMS
   // SVG Collections expose numberOfItems rather than length.
   int get length => numberOfItems;
 $endif
 
 $if DEFINE_LENGTH_SETTER
   set length(int value) {
     throw new UnsupportedError("Cannot resize immutable List.");
   }
 $endif
 
   $E get first {
     if (this.length > 0) {
 $if DART2JS
-      return JS('$E', '#[0]', this);
+      return JS('$EJS', '#[0]', this);
 $else
 $if USE_NATIVE_INDEXED_GETTER
       return $GETTER(0);
 $else
       return this[0];
 $endif
 $endif
     }
     throw new StateError("No elements");
   }
 
   $E get last {
     int len = this.length;
     if (len > 0) {
 $if DART2JS
-      return JS('$E', '#[#]', this, len - 1);
+      return JS('$EJS', '#[#]', this, len - 1);
 $else
 $if USE_NATIVE_INDEXED_GETTER
       return $GETTER(len - 1);
 $else
       return this[len - 1];
 $endif
 $endif
     }
     throw new StateError("No elements");
   }
 
   $E get single {
     int len = this.length;
     if (len == 1) {
 $if DART2JS
-      return JS('$E', '#[0]', this);
+      return JS('$EJS', '#[0]', this);
 $else
 $if USE_NATIVE_INDEXED_GETTER
       return $GETTER(0);
 $else
       return this[0];
 $endif
 $endif
     }
     if (len == 0) throw new StateError("No elements");
     throw new StateError("More than one element");
   }
 
   $E elementAt(int index) => this[index];
   // -- end List<$E> mixins.