Check "Bringing SIMD to the Web via Dart" by John McCutchan:

https://www.youtube.com/watch?v=CKh7UOELpPo

He's also the guy wrote the proposal for ECMAScript:

https://github.com/johnmccutchan/ecmascript_simd

Also, using SIMD is way easier than using shaders. You just write something like this:

  double average (Float32x4List list) {
    var n = list.length;
    var sum = new Float32x4.zero();
    for (int i = 0; i < n; i++) {
      sum += list[i];
    }
    var total = sum.x + sum.y + sum.z + sum.w;
    return total / (n * 4);
  }

Instead of:

  double average (Float32List list) {
    var n = list.length;
    var sum = 0.0;
    for (int i = 0; i < n; i++) {
      sum += list[i];
    }
    return sum / n;
  }