add-to-app cases adding Flutter as not a single contiguous navigation segment will want to preserve multiple Flutter instances' states and navigations stacks such as 2 Flutter tabs or a Flutter->native->Flutter navigation stack.

This involves first fleshing out the details of #37643 and deciding on a multi engine approach or a single engine approach.

This will likely be scheduled for 2020. Until then, using something like https://github.com/alibaba/flutter_boost (a single engine approach) seems reasonable.