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This CL changes imports from iron-icons.html to iron-icon.html (without S),
as well as moving element imports to where it's used.

Bug: 855205
Change-Id: Ibe22c21ec9270258efafd1e06fed73de1d226f2d
Reviewed-on: https://chromium-review.googlesource.com/1145598
Commit-Queue: Scott Chen <scottchen@chromium.org>
Reviewed-by: Yue Li <updowndota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577284}