ProcessTermError instance isn't an instance of ProcessTermError

[alecmev]

Do you want to request a feature or report a bug?

This is a bug.

What is the current behavior?

Running yarn -s run false with the following package.json:

{ "scripts": { "false": "false" } }

Results in the following output:

error An unexpected error occurred: "Command failed.
Exit code: 1
Command: sh
Arguments: -c false
Directory: /my/cwd
Output:
".

What is the expected behavior?

The expected output:

error Command failed with exit code 1.

Please mention your node.js, yarn and operating system version.

Node.js: 9.7.1
Yarn: 1.5.1
OS: Linux 4.15.6

Extra info.

I'm using --silent for brevity. I've inspected the compiled sources, inserted some logs around this catch, and narrowed it down to this fact:

new ProcessTermError('whatever') instanceof ProcessTermError === false

Appears to be some sort of an issue with Babel/Webpack configuration, and I'm assuming that other uses of instanceof are affected by this too.

[alecmev]

Fixed by #5497.