utf converter seems to be slow

[floitschG]

According to http://stackoverflow.com/questions/28359949/how-to-convert-binary-data-to-utf8-string-after-dart2js utf8 encoding/decoding is slow in dart2js.

Load:

---

import 'dart:convert';
import 'package:archive/archive.dart';
import 'package:crypto/crypto.dart';

var base64 = CryptoUtils.base64StringToBytes(window.localStorage[storageName]);
var decompress = new GZipDecoder().decodeBytes(base64);
storage = JSON.decode(new Utf8Decoder().convert(decompress));

---

Save:

---

var g = JSON.encode(storage);
List<int> bz2 = new GZipEncoder().encode(new Utf8Encoder().convert(g));
window.localStorage[storageName] = CryptoUtils.bytesToBase64(bz2);

---

[rakudrama]

Consider using TextEncoder / TextDecoder on browsers that support this operation.
It might still be necessary to fall back on the current implementation if there is a mismatch in error handling.

[rakudrama]

3836c70 uses the browser's TextDecoder where it is available and the input does not contain any tricky situations.
Encoding is still slow.