Environments:

• Prettier Version: 1.19.0
• Running Prettier via: CLI
• Runtime: Node v10.14
• Operating System: MacOs

Steps to reproduce:
I did a clean install in a new directory of prettier:

$ yarn install prettier
$ cat test.js | prettier --stdin --stdin-filepath test.js
[error] regeneratorRuntime is not defined

Seems like the built "binaries" of node_modules/prettier/bin-prettier.js node_modules/prettier/index.js and node_modules/prettier/third-party.js all have this undefined name regeneratorRuntime presumably from a babel generator transformation, probably only used during stream/stdin reading. I double checked that 1.18.2 does not have this issue.