This issue was originally filed by @tatumizer

Please run the following program (I tested on Windows, not sure if Linux has the same effect).

import 'dart:typed_data';
int iterations=10000000;
testIndexOf8() {
  var list=new Uint8List(8);
  int x=0;
  for (int i=0; i<iterations; i++) {
      x|=list.indexOf(1);
  }
  return x;
  
}
testIndexOf16() {
  var list=new Uint16List(8);
  int x=0;
  for (int i=0; i<iterations; i++) {
      x|=list.indexOf(1);
  }
  return x;
}

main() {
  var w;
  testIndexOf8(); // warmup
  testIndexOf16(); // warmup
  w = new Stopwatch()..start();
  print("result=${testIndexOf8()}, time=${w.elapsedMilliseconds}");
  
  w.reset();
  print("result=${testIndexOf16()}, time=${w.elapsedMilliseconds}");
}

printout:
result=-1, time=850
result=-1, time=884

NOTE: I print "result" just to eliminate very unlikely case where compiler can optimize stuff away.

Now let's conduct a number of experiments

1. comment out both warmup calls:
     result=-1, time=251 -- YES! It can run by 4 times faster!
     result=-1, time=935
2. with warmup calls both commented out, go to testIndexOf16 and replace Uint16List with Uint8List constructor:
     result=-1, time=290
     result=-1, time=240
     
   And so on. I managed to get all possible combinations of ~200, ~900 milliseconds by randomly commenting/uncommenting stuff.
     
   How one method affects the other is a mystery. One gets deoptimized because the other one gets optimized? Or one gets optimized because the other one gets de-optimized? But sometimes they are both optimized, and at other times both de-optimized? I admit I wasn't able to discover a pattern.