Originally, my aim was just to refactor (as per usual), but while messing around with the `TableBorder.symmetric` constructor, I realized that `borderRadius` was missing!
This pull request makes a few class constructors more efficient, and it fixes#144277 by adding the missing parameter.
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Fixes in preparation for https://dart-review.googlesource.com/c/sdk/+/330561.
The change referenced above tightens the `use_build_context_synchronously` to ensure that `mounted` is checked on the appropriate `BuildContext` or `State`. This change fixes up the flutter/flutter in preparation of this new enforcement.
Move Flutter Gallery to dev/integration_tests/ as it is an older copy used only for testing. The current version of the Flutter Gallery now lives in https://github.com/flutter/gallery.
