Add a percent-encoding converter.

lib/src/percent.dart

Index: lib/src/percent.dart
diff --git a/lib/src/percent.dart b/lib/src/percent.dart
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+++ b/lib/src/percent.dart
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+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library convert.percent;
+
+import 'dart:convert';
+
+import 'percent/encoder.dart';
+import 'percent/decoder.dart';
+
+export 'percent/encoder.dart' hide percentEncoder;
+export 'percent/decoder.dart' hide percentDecoder;
+
+/// The canonical instance of [PercentCodec].
+const percent = const PercentCodec._();
+
+// TODO(nweiz): Add flags to support generating and interpreting "+" as a space
+// character.

[Lasse Reichstein Nielsen, 2015/10/08 10:30:36]

That's not plain percent-encoding. It's www-form-urlencoding which is its own
encoding. Percent-encoding per RFC 3986 (2.1 Percent-Encoding) doesn't do that.
I'd probably make a separate codec called wwwFormEncoding.

RFC 3986 does say that it encodes characters that have meaning in the context,
so you may want to either specify a context or allow the user to specify it.
A context is basically a set of allowed ASCII characters.
Defaulting to "query component" is probably a good start, that's where you will
need it.

+/// A codec that converts byte arrays to and from percent-encoded (also known as
+/// URL-encoded) strings according to [RFC 3986][rfc].
+///
+/// [rfc]: https://tools.ietf.org/html/rfc3986#section-2.1
+///
+/// [encoder] encodes all bytes other than ASCII letters, decimal digits, or one
+/// of `-._~`. This matches the behavior of [Uri.encodeQueryComponent] except
+/// that it doesn't encode `0x20` bytes to the `+` character.
+///
+/// To be maximally flexible, [decoder] will decode any percent-encoded byte and
+/// will allow any non-percent-encoded byte other than `%`. By default, it
+/// interprets `+` as `0x2B` rather than `0x20` as emitted by
+/// [Uri.encodeQueryComponent].
+class PercentCodec extends Codec<List<int>, String> {
+  PercentEncoder get encoder => percentEncoder;
+  PercentDecoder get decoder => percentDecoder;
+
+  const PercentCodec._();
+}