Dart support

This will add support for [Dart](https://www.dartlang.org/).

```yaml
language: dart
```

You can provide the version to use with the `dart` property. `stable` is the default and you can use either `dev` or a version number.

If the project contains a `pubspec.yaml` file then `pub get` is launch.

Finally the script run `dart test/all_test.dart` if `test/all_test.dart` exists.
Questions about the instance set up:

- is `unzip` available ?
- is `GLIBC` available ?

Ideally could you try the following script and ensure it works correctly:

```shell
curl http://storage.googleapis.com/dart-archive/channels/stable/release/latest/sdk/dartsdk-linux-x64-release.zip > dartsdk-linux-x64-release.zip
unzip dartsdk-linux-x64-release.zip > /dev/null
rm dartsdk-linux-x64-release.zip
export DART_SDK="$PWD/dart-sdk"
export PATH="$DART_SDK/bin:$PATH"
dart --version
```