Build Fragments without the RECONSTRUCT_FLOW bit
The RECONSTRUCT_FLOW restyle damage bit shouldn't apply to newly
constructed flow tree elements. It is explicitly removed for Flows, but
not for Fragments. This causes RECONSTRUCT_FLOW to bubble up to Flows
that contain any Fragments at all. Instead explicitly remove the
RECONSTRUCT_FLOW bit when creating a new Fragment.
There isn't a good way to test this currently, but all tests should
continue to pass.
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