webpack.DefinePlugin and process.env.DEBUG

My webpack config:
```
...
plugins: [
  new webpack.DefinePlugin({
    'process.env': {
      NODE_ENV: JSON.stringify('production'),
      DEBUG: true
    }
  })
],

target: 'electron-main',
...
```

So webpack changes
`process.env.DEBUG = namespaces;`
to
`true = namespaces;`
which leading to error
`ReferenceError: Invalid left-hand side in assignment`