Support for MutableCapabilities and ImmutableCapabilities

**Problem**: After updating from Selenium v3.6.0 to Selenium v3.8.1, the capabilities don't appear to work as expected. For example, downloads are no longer going to`/shared/exports` using the snippet below.
```
Map capabilities = {
'platform': 'LINUX',
'browserName': 'chrome',
'chromeOptions': {
'args': ['window-size=1280,600', 'disable-infobars', 'enable-precise-memory-info'],
'prefs': {
'download': {'default_directory': '/shared/exports/', 'prompt_for_download': 'false'}
}
}
}
driver = await io.createDriver(desired: capabilities, uri: seleniumUri);
```

- The version of Dart on your system: Dart VM version: 1.24.2 (Thu Jun 22 15:42:21 2017) on "linux_x64"
- The operating system you are running: linux_x64
- The version of the webdriver package you are using: 1.2.3

I assume the web driver implementation will need to support MutableCapabilities and ImmutableCapabilities?