A new feature flag has been added, debugRenderPhaseSideEffectsForStrictMode. When enabled, StrictMode subtrees will also double-invoke lifecycles in the same way as debugRenderPhaseSideEffects.
By default, this flag is enabled for __DEV__ only. Internally we can toggle it with a GK.
This breaks several of our incremental tests which make use of the noop-renderer. Updating the tests to account for the double-rendering in development mode makes them significantly more complicated. The most straight forward fix for this will be to convert them to be run as internal tests only. I believe this is reasonable since we are the only people making use of the noop renderer.