The main reason is to do this change is to avoid inconsistencies in what is considered to be a `JSArray`. Using `Array.isArray` allows js-interop where the JavaScript code subclasses `Array`.
dart2js-production results:
ArrayLoop.pseudopoly.hoisted1-indexing -50.20%
ArrayLoop.pseudopoly.indexing -35.18%
Iteration.concat.manual 11.69%
ListCopy.List.int.unmodifiable.2 12.03%
ListCopy.for.int.2 12.58%
ListCopy.spread.int.2 13.57%
ListCopy.toList.fixed.100 16.06%
ListCopy.List.of.fixed.100 16.96%
ListCopy.toList.100 17.28%
ListCopy.spread.int.cast.2 18.45%
ObjectHash.hash.5 19.62%
TypedDataPoly.A_UVx5.view.2 21.12%
ObjectHash.manual.5 21.70%
ListCopy.spread.int.map.2 21.96%
ListCopy.spread.int.cast.100 22.12%
TypedDataPoly.A_UVx5.view.100 23.04%
ListCopy.spread.int.map.100 25.26%
ListCopy.List.of.100 27.88%
ImagingGaussianBlurOnce 29.67%
MegaEquality 37.90%
An investigation of the two regressions shows that they benefit from a micro-benchmarking effect. The previous code was monomorphic at the property access in `receiver.constructor == Array`. If this is forced to be polymorphic, the baseline is quite a bit worse, leading to an improvement in line with some of the other benchmarks.
Change-Id: I5c265b1d7408fbd41da9c6fa17472bf648000c8d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/287140
Reviewed-by: Nate Biggs <natebiggs@google.com>
Commit-Queue: Stephen Adams <sra@google.com>
Stephen Adams
62a67ba5bc