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.

首先，在 Node 5.4 下以及经过 webpack 打包后，使用 MyClass/EventEmitter 继承代码片段复现该问题。比较两种环境中得到的 MyClass 原型；当自定义的 foo 方法与继承的 EventEmitter 方法同时保持可用时，即表示完成。

由索引模型根据 Issue 内容生成。