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This is needed so that pages using the polyfill will still work with
Polymer 1. Polymer 1 is currently tested on the bots and is expected
to be supported through at least M-74.

Bug: 925517
Change-Id: Iabe257fb1228dd9803403dd478889080bba64962
Reviewed-on: https://chromium-review.googlesource.com/c/1484506
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635888}