Closes https://github.com/dart-lang/sdk/issues/41699
createFragment should accept a nullable html string in its
implementation. As such, usages of createFragment within dart:html
are modified as well so the html string does not have to be
non-nullable.
Change-Id: Ia04ad2da9e2a48cabda8774f0af75559fb03a7d2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/145570
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Reviewed-by: Nicholas Shahan <nshahan@google.com>
Commit-Queue: Srujan Gaddam <srujzs@google.com>
