Behaviour of util.promisify(childProcess.exec) inconsistent with node

[arantes555]

• Electron Version: 1.8.7 & 2.0.3
• Operating System (Platform and Version): MacOS 10.13.5
• Last known working Electron version: None

Expected Behavior
util.promisify(childProcess.exec) should behave the same on electron as on node

Actual behavior
On node, promisify-ed versions of childProcess.exec & childProcess.execFile have custom behaviour, resolving to an object of the form {stdout, stderr}, or rejecting with an Error with the custom properties stdout & stderr. (Reference : https://nodejs.org/api/child_process.html#child_process_child_process_exec_command_options_callback)

On electron, there is no such behaviour, which leads to weird stuff.

To Reproduce

git clone https://github.com/arantes555/electron-bug-child_process.git -b master
cd electron-bug-child_process
npm install

# electron@1.8.7
npm run start-node # Note the objects `{ stdout: 'foobar\n', stderr: '' }` printed here
npm run start-electron # Note the simple `foobar`s printed here

# electron@2.0.3
git checkout electron2
npm install
npm run start-node
npm run start-electron # Note the same behaviour

Additional Information
The node behaviour is achieved thanks to https://nodejs.org/api/util.html#util_custom_promisified_functions . On node, childProcess.exec[util.promisify.custom] is defined, whereas it is undefined on electron.

It can also be noted that this issue has already been (poorly) reported some time ago #10925

[miniak]

@arantes555 ASAR support integration code is wrapping childProcess.exec, therefore childProcess.exec[util.promisify.custom] is being lost. I am fixing that in #13845