Unable to upload files to remote WebDrivers

We are not able to upload files on file input elements on remote services like Sauce Labs.
This is normally done by sending keys to the element. The Dart client doesn't appear to upload the file to remote services first like the others do. [Python Example](https://github.com/SeleniumHQ/selenium/blob/0e67b7b688b505173264d4ba552b110a1985475b/py/selenium/webdriver/remote/webelement.py#L303-L308)

If the keys being sent to an element are found to be a path that points to a local file, the file is upload to the WebDriver with a POST request to `/session/$sessionId/file`, and the response to that request is what's actually sent to the element.
- Dart version: 1.9.0-dev.8.0
- OS: OS X 10.10.3
- Dart webdriver version: 0.9.1