Also, make hasOneLineDescription more discerning.
Also, add a test for hasOneLineDescription.
Also, add a test for GravitySimulation, to test the toString.

```dart
expect(x, moreOrLessEquals(0.0));
```

...for those cases where `x` might be 1e-11 or whatever.

Also, be more resilient when dumping the tree from inside the test framework.
Also, add an assert that helped me debug something the other day.