This should be do-able now that  issue 908045  is complete, and the mock progress center has been added for closure compilation [1].

[1] https://bugs.chromium.org/p/chromium/issues/detail?id=907962#c7

Nicking :)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1a127741963596af0097658c733657fc0a97e5fe

commit 1a127741963596af0097658c733657fc0a97e5fe
Author: Noel Gordon <noel@chromium.org>
Date: Tue Nov 27 08:07:08 2018

Add mock progress center items count helper for unittests

Add helper to count the number of items on the MockProgressCenter. Use
it in drive_sync_handler_unittest as an example, to remove a shed-load
of closure compiler complaints about the following code:

   Object.keys(progressCenter.items)

Move that code into the MockProgressCenter helper: getItemCount(), and
the Closure compiler will be a happy koala.

No change in test behavior, no new tests.

Bug:  907964 
No-try: true
Change-Id: I3da5fda8226a64a0cad27dcb57b84fac6c7a43be
Reviewed-on: https://chromium-review.googlesource.com/c/1351201
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Stuart Langley <slangley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611038}
[modify] https://crrev.com/1a127741963596af0097658c733657fc0a97e5fe/ui/file_manager/file_manager/background/js/drive_sync_handler_unittest.js
[modify] https://crrev.com/1a127741963596af0097658c733657fc0a97e5fe/ui/file_manager/file_manager/background/js/mock_progress_center.js

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8acca942bf7975a8da43c1026229a172c50970d0

commit 8acca942bf7975a8da43c1026229a172c50970d0
Author: Noel Gordon <noel@chromium.org>
Date: Tue Nov 27 10:47:22 2018

Minor code / comment fixes for drive_sync_handler_unittest

 - update the comments to match what the actually code does
 - check progress center item state like other unittests do

No change in test behavior.

Bug:  907964 
Tbr: slangley
No-try: true
Change-Id: I8f307b699d5005cbdefe346d838088610c9f88cf
Reviewed-on: https://chromium-review.googlesource.com/c/1352072
Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611065}
[modify] https://crrev.com/8acca942bf7975a8da43c1026229a172c50970d0/ui/file_manager/file_manager/background/js/drive_sync_handler_unittest.js

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b5c8517a7d71c4c9cc1e867f2c5aea9ff3b5a746

commit b5c8517a7d71c4c9cc1e867f2c5aea9ff3b5a746
Author: Noel Gordon <noel@chromium.org>
Date: Tue Nov 27 21:45:14 2018

Closure compile drive_sync_handler_unittest

 - remove drive_sync_handler_unittest.html
 - add BUILD rules to auto-generate and compile this unit test
 - BUILD: progress center does depends on cr.EventTarget
 - BUILD: drive sync handler does depend on AsyncUtil.Queue
 - define and use a mockChrome, and install it during tests
 - fix Closure compile errors

Bug:  907964 
Change-Id: I464f4fb43e2ab145f1a6c85b53f8292a8a2c69fc
Reviewed-on: https://chromium-review.googlesource.com/c/1352113
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Stuart Langley <slangley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611345}
[modify] https://crrev.com/b5c8517a7d71c4c9cc1e867f2c5aea9ff3b5a746/chrome/browser/chromeos/file_manager/file_manager_jstest.cc
[modify] https://crrev.com/b5c8517a7d71c4c9cc1e867f2c5aea9ff3b5a746/ui/file_manager/file_manager/background/js/BUILD.gn
[delete] https://crrev.com/a9d0311e3b93a1f05865446252da9046f1045157/ui/file_manager/file_manager/background/js/drive_sync_handler_unittest.html
[modify] https://crrev.com/b5c8517a7d71c4c9cc1e867f2c5aea9ff3b5a746/ui/file_manager/file_manager/background/js/drive_sync_handler_unittest.js

^^^ No change in test behavior.