Hint for when you export an identifier twice

[pq]

From @Hixie on June 21, 2016 17:1

export 'package:flutter/rendering.dart' show
    Axis,
    BoxConstraints,
    CrossAxisAlignment,
    CustomClipper,
    CustomPainter,
    FixedColumnCountGridDelegate,
    Axis,
    ViewportDimensionsChangeCallback;

...did not cause an error (note the duplicate Axis).

Copied from original issue: dart-lang/linter#263

[pq]

From @srawlins on May 19, 2018 4:6

I think this should even be a hint. I don't think there would be any false positives.

[pq]

From @bwilkerson on May 19, 2018 4:19

I agree. It's another form of dead code so it should be a hint.

[pq]

@bwilkerson : should we move this over to the SDK repo?

[pq]

From @bwilkerson on May 20, 2018 14:7

Yes.

[srawlins]

What about across import/export directives?

import 'dart:async';
import 'dart:async' show Future;

Future<void> main() async {
  print('hey');
}

I'd expect a warning on the second line, "Future is already imported on line 1."

[bwilkerson]

Or we should flag the first line if Futture is the only identifier being used from async :-)

There's always a trade-off. If users are seeing this error frequently enough that the added checks would be helpful, then we should do it. If not, then we aren't necessarily doing are users a favor by (a) taking the time to implement this rather than some more useful functionality and (b) spending their processor's cycles to confirm that there are no problems. I have no data for how helpful such a check would be.

[srawlins]

Landed!