Inconsistent behaviour with node inherits

I have a node class that inherits Event emitter:

```
function MyClass() {
EventEmitter.call(this);
}
util.inherits(MyClass, EventEmitter);
```

This class also has it's prototype object with methods:

```
MyClass.prototype.foo = function() {};
```

When I run my code on node (5.4) everything runs smoothly, but once webpack'ed the MyClass prototype gets squashed and only the EventEmitter methods survive.