
Towards https://github.com/flutter/flutter/issues/164315. This PR just writes `bin/cache/engine.stamp` identically to how `bin/internal/engine.version` would otherwise be written, with a caveat that _if_ `engine.version` is tracked, it is now _copied_ to `bin/cache/engine.stamp`. After this lands, I'll send PRs to update tooling that looks for `engine.version` and give a heads up to the larger team (i.e. Dart HH bot or whomever we will break by doing this).