feat: Upload/download files to google cloud storage.

This change set adds the ability to upload or download files from google cloud storage.
Should be relative import. Try the following 

```
// In `lib/lib_a.dart`
import 'package:sandbox/lib_b.dart';
void foo(Value v) {
   print(v.value);
}
```

```
// In file lib/lib_b.dart
class Value {
   final value = 4;
}
```
```
// In file bin/main.dart
import '../lib/lib_a.dart';
import '../lib/lib_b.dart';

void main() {
   foo(new Value());
}
```

The lesson: Do not use package imports for files in the same library.