anchor.diff:        top = Directory.current.path;
       }
       return top;
  +  }
  +
  +  static String getDartSdkRevision(String top) {
  +    top = getTop(top);
  +    ProcessResult res =
  +        Process.runSync('git', ['--git-dir=$top/.git', 'rev-parse', 'HEAD']);
  +    if (debug) {
  +      print('\ngit rev-parse HEAD result:\n'
  +          '${res.exitCode}\n${res.stdout}\n');
  +    }
  +    return res.stdout;
     }
   
     // Picks a mode (command line or random).
  
positive.code:   final String mode1;
    final String mode2;
    final bool rerun;
  
    // Test.
    Random rand;
  
negative.code:       this.top,
        this.mode1,
        this.mode2,
        this.rerun,
        this.dartSdkRevision);
  
    int run() {
      setup();
  