perf: change StringBuffer for String

```
const _LEN = 50000;

sb() {
  var sw = new Stopwatch()..start();

  for (var j = 0; j < _LEN; j++) {
    var sb = new StringBuffer("a");
    sb.write("b");
    sb.write("c");
    sb.write("d");
    sb.write("e");
    sb.write("f");
    sb.toString();
  }

  sw.stop();
  print("sb: ${sw.elapsedMicroseconds / 1000}");
}

direct() {
  var sw = new Stopwatch()..start();

  for (var j = 0; j < _LEN; j++) {
    String sb = "a";
    sb += "b";
    sb += "c";
    sb += "d";
    sb += "e";
    sb += "f";
  }

  sw.stop();
  print("string: ${sw.elapsedMicroseconds / 1000}");
}

main() {
  sb();
  direct();
}
```

sb: 34.648
string: 26.487