Hide this behind a flag, so that we can disable it when building in the flutter in-tree build.
The flag will be set to false for the in-tree flutter build, see
https://github.com/flutter/engine/pull/40195
Change-Id: I248376985d05bfb248a2eab6fa377cbb01d75654
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/288040
Commit-Queue: Jackson Gardner <jacksongardner@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>