dart format with --enable-experiment=variance removes variance annotations

[plammens]

Hit a bug in the formatter with dart format --enable-experiment=variance file.dart.
The formatter produced unexpected output. Input was:

class Consumer<in T> {
  void consume(T value) {
    print("Consumed: $value");
  }
}

Which formatted to:

class Consumer<T> {
  void consume(T value) {
    print("Consumed: $value");
  }
}

(The type parameter variance annotation was removed.)