record that Gamepad.buttons creates GamepadButtons

tools/dom/templates/immutable_list_mixin.darttemplate

Index: tools/dom/templates/immutable_list_mixin.darttemplate
diff --git a/tools/dom/templates/immutable_list_mixin.darttemplate b/tools/dom/templates/immutable_list_mixin.darttemplate
index b9820dcb071bb781ad55e9f00eccae6ce4590260..02b0b4e0683e267de4a32e4d26317d35547b495e 100644
--- a/tools/dom/templates/immutable_list_mixin.darttemplate
+++ b/tools/dom/templates/immutable_list_mixin.darttemplate
@@ -15,7 +15,7 @@ $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);
@@ -31,7 +31,7 @@ $endif
     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);
@@ -47,7 +47,7 @@ $endif
     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);