I noticed that the warning now says that /deep/ is a no-op

"[Deprecation] /deep/ combinator is no longer supported in CSS dynamic profile. It is now effectively no-op, acting as if it were a descendant combinator. You should consider to remove it. See https://www.chromestatus.com/features/4964279606312960 for more details."

@dbeam: I am wondering if this is contributing to any newly found test flakiness.

FWIW, here is a list of places where disableAnimationsAndTransitions() is currently used at 

chrome/browser/ui/webui/extensions/extension_settings_browsertest.js
chrome/test/data/webui/print_preview/print_preview_ui_browsertest.js
chrome/test/data/webui/settings/android_apps_page_test.js
chrome/test/data/webui/settings/bluetooth_page_tests.js
chrome/test/data/webui/settings/device_page_tests.js
chrome/test/data/webui/settings/internet_page_tests.js
chrome/test/data/webui/settings/languages_page_tests.js

We should probably come up with a plan on how to disable animations without using /deep/.

+rbpotter as FYI, since one of those tests is print_preview_ui_browsertest.js

Fix candidate at https://codereview.chromium.org/2927663003/.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2deb8c3767fbb87c7fac334eda0624bc838903df

commit 2deb8c3767fbb87c7fac334eda0624bc838903df
Author: dbeam <dbeam@chromium.org>
Date: Thu Jun 08 00:38:36 2017

Fix Test.disableAnimationsAndTransitions to work with new /deep/

R=hcarmona@chromium.org
BUG=724597

Review-Url: https://codereview.chromium.org/2927663003
Cr-Commit-Position: refs/heads/master@{#477834}

[modify] https://crrev.com/2deb8c3767fbb87c7fac334eda0624bc838903df/chrome/test/data/webui/test_api.js

It's worth noting that the previous code would apply for all elements that are created during the lifetime of the test. Whereas the current solution will only affect elements that are already in the DOM. I don't know if we plan to address this (or defer until it proves to be an issue).

right, potential ways to do do this would be

1) listen for dom-change events (for polymer code, assuming they're on)
2) use a MutationObserver

MutationObserver observes a single HTMLElement and only reports DOM changes of its immediate children AFAIK.

https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver#MutationObserverInit childList

yeah, sorry, you're right: I thought childList was recursive.

This is a no longer blocking issue. /deep/ and ::shadow were removed at M63.