Bug Report

• Package version(s):
  Latest

• Browser and OS versions:
  IE11

Priorities and help requested (not applicable if asking question):

Are you willing to submit a PR to fix?
Yes

Requested priority: (Blocking, High, Normal, Low)
Normal

Products/sites affected: (if applicable)
VSTS

Describe the issue:

Focus outline is not visible on buttons in IE11 (Any button extending BaseButton component)

The issue is more visible in icon button

In chrome it looks like this -

It seems like this change caused this regression-
8a8c240#diff-2ba1f2ae618d0fef86a4c03a08260b38

Actual behavior:

Focus outline is not visible in IE11

Expected behavior:

Focus outline on buttons should be visible in IE11

If applicable, please provide a codepen repro:

Can be reproed in localhost site from office fabric react repo. Note that this issue is not visible in https://developer.microsoft.com/en-us/fabric#/components/button because this page adds a rule -
button {
overflow: visible
}
via a non-fabric css which fixes this issue.