Dart 2.0 support

Hey,

since strong_mode is working I was thinking Dart 2 is already supported, but if I try to pub get `^1.0.0` of parsers I'll get:
```
Resolving dependencies...
The current Dart SDK version is 2.0.0.

Because systori depends on parsers >=0.0.2 which requires SDK version <2.0.0, version solving failed.
```