hive_generator: ^0.7.0+2 fails resolution with Flutter Beta

**Steps to Reproduce**

New flutter project using Flutter beta (1.7.0). Add

```
hive: ^1.4.1+1
hive_flutter: ^0.3.0+2
hive_generator: ^0.7.0+2 (dev dep)
```

Run **flutter test**.

Resolution dependency fails.


Workaround to get tests to run:

Set hive_generator: dependency to be open, and it will resolve to hive_generator: 0.5.2

**Version**

```
hive: ^1.4.1+1
hive_flutter: ^0.3.0+2
```