invariant_booleans: false positive on comparisons

invariant_booleans reports the `else if` condition of the following code:
```dart
void f(int a, int b) {
  if (a < b) {
  } else if (b < a) {}
}
```

whereas this is correctly not reported:
```dart
void f(int a, int b) {
  if (a < b) {
  } else if (a > b) {}
}
```

I'm using the linter that is bundled with Dart 2.0.0-dev.66.0.