Consider these two benchmarks:

class PerfNowBenchmark extends BenchmarkBase {
  const PerfNowBenchmark() : super("PerfNow");

  // The benchmark code.
  void run() {
    var time = window.performance.now();
  }
}

class StopwatchBenchmark extends BenchmarkBase {
  Stopwatch sw;
  
  StopwatchBenchmark() : super("Stopwatch") {
    sw = new Stopwatch()..start();
  }

  // The benchmark code.
  void run() {
    var time = sw.elapsedMicroseconds;
  }
  
}

(using the benchmark_harness package)

When run in Dartium:

PerfNow(RunTime): 6.421432172017325 us.
Stopwatch(RunTime): 4.861614153131122 us.

When run in dart2js:

PerfNow(RunTime): 3.4266818582895717 us.
Stopwatch(RunTime): 5.462122908689691 us.

 
Here is a simple main() to drive it:

import 'dart:html';
import 'package:benchmark_harness/benchmark_harness.dart';

void main() {
  new PerfNowBenchmark().report();
  new StopwatchBenchmark().report();
}