BigInt.toDouble() fails to convert large negative values to -Infinity. Instead, it produces Infinity for both positive and negative values. A small repro:

void main() {
  BigInt positive = BigInt.from(10).pow(309);
  BigInt negative = -positive;
  print(positive.toDouble()); // Correctly produces `Infinity`.
  print(negative.toDouble()); // Incorrectly produces `Infinity` instead of `-Infinity`.
}

This breaks constant folding in dart2js and may cause issues elsewhere. (Confirmed this behavior for the VM and dart2js backends; unsure about others.)

Dart VM version: 2.2.0-dev.2.1+google3-v2.2.0.dev.2.1 (google3) on "linux_x64"