anchor.diff:        this.fileName, List<String> extraFlags) {
       description = '$prefix-$tag';
       dart = '$top/out/$tag/dart';
  -    cmd = [dart] + extraFlags + [fileName];
  +    cmd = [dart, ...extraFlags, fileName];
     }
   
     TestResult run() {
  
positive.code: /// Command runner.
  TestResult runCommand(List<String> cmd, Map<String, String> env) {
    ProcessResult res = Process.runSync(
        'timeout', ['-s', '$sigkill', '$timeout'] + cmd,
        environment: env);
    if (debug) {
      print('\nrunning $cmd yields:\n'
  
negative.code:     snapshot = '$tmp/snapshot';
      env = Map<String, String>.from(e);
      env['DART_CONFIGURATION'] = tag;
      cmd = [precompiler, ...extraFlags, fileName, snapshot];
    }
  
    TestResult run() {
  