anchor.diff:        Map<String, String> env, String fileName, Random rand) {
       String prefix = mode.substring(0, 3).toUpperCase();
       String tag = getTag(mode);
  -    // Prepare extra flags.
       List<String> extraFlags = [];
  +    // Required extra flags for kbc.
       if (mode.startsWith('kbc-int')) {
         prefix += '-INT';
         extraFlags += [
  
positive.code:           prefix, tag, top, tmp, env, fileName, extraFlags);
      } else if (mode.startsWith('kbc')) {
        return new TestRunnerKBC(
            prefix, tag, top, tmp, env, fileName, extraFlags);
      } else if (mode.startsWith('djs')) {
        return new TestRunnerDJS(prefix, tag, top, tmp, env, fileName);
      }
  
negative.code:     } else if (mode.startsWith('kbc-cmp')) {
        prefix += '-CMP';
        extraFlags += ['--use-bytecode-compiler'];
      }
      // Every once in a while, go directly from source for kbc-*-x64.
      bool kbcSrc = false;
      if (mode.startsWith('kbc') &&
          mode.endsWith('x64') &&
          rand.nextInt(4) == 0) {
        prefix += '-SRC';
        kbcSrc = true;
      }
      // Every once in a while, stress test JIT.
      if (mode.startsWith('jit') && rand.nextInt(4) == 0) {
  