In `javascript` we can do something like this : 

```
var x = {
  'name' : 'john',
  'modifiedName' : function () { return this.name + ' <- modified'}
};

```

so x.modifiedName() returns `john <- modified`

How to do that on `dart` ? 

I want to use some of the value from a `Map` to return a value for a key, like the `javascript` example.