compiler: Remove unused system import
compiler: Remove unused system import

Change-Id: I98fc091a7f2a701f3f325543aeecd451120a6ae7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/189660
Reviewed-by: Sigmund Cherem <beaea8fd7d2a56555046ba4f362c8a93e3938576@google.com>
Commit-Queue: Samuel Rawlins <5cf2cac5eec2ad6d0eb2b0f4ff6af9a590ce2bf6@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk