Fix 18858: Link to docs built by dartdocs.org

Fix http://dartbug.com/18858
Python is foreign to me :-(

Does the following code (copy-paste powered) work please?

```python
    @property
    def documentation(self):
        """The documentation URL for the package, or None."""
        if self.latest_version is None: return None
        return self.latest_version.pubspec.get('documentation',
            'http://www.dartdocs.org/documentation/%s/%s/index.html#%s' % \
            (self.name, self.latest_version.version, self.name))
```