we can't use {**dict1, **dict2} in python2 code.
see report https://travis-ci.com/bmsdave/v8/jobs/180113432

Bug: v8:935200
Change-Id: I2f2a1283583432d6d0167436dd7d4205455049cb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1484303
Commit-Queue: Sergiy Belozorov <sergiyb@chromium.org>
Reviewed-by: Sergiy Belozorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60039}