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Trying to run a composer install on the 2.7 branch and I get:
Installing core node_modules
yarn install v1.12.3
[1/4] Resolving packages...
[2/4] Fetching packages...
error https://registry.yarnpkg.com/flatmap-stream/-/flatmap-stream-0.1.1.tgz: Extracting tar content of undefined failed, the file appears to be corrupt: "Unexpected end of data"
info Visit https://yarnpkg.com/en/docs/cli/install for documentation about this command.
Installing core node_modules failed
So the package in question has been removed from yarn due to a security issue. See https://github.com/yarnpkg/yarn/issues/6725#issuecomment-442395010.