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This ensures that that token is always at the end of
a token chunk. This is important because the parser can
only yield or pause at token chunk boundaries. Pausing
may be forced by a custom element because its createdCallback
routine may add a stylesheet, which blocks the HTML parser.

Bug: 904966

Change-Id: I9252067f435d048b860cc9725ba23d829c9f644d
Reviewed-on: https://chromium-review.googlesource.com/c/1351366Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611317}