Add a percent-encoding converter.

lib/src/hex/encoder.dart

 // 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.hex.encoder;
 
 import 'dart:convert';
 import 'dart:typed_data';
 
 import 'package:charcode/ascii.dart';
@@ skipping 57 matching lines @@
     buffer[bufferIndex++] = _codeUnitForDigit((byte & 0xF0) >> 4);
     buffer[bufferIndex++] = _codeUnitForDigit(byte & 0x0F);
   }
 
   if (byteOr >= 0 && byteOr <= 255) return new String.fromCharCodes(buffer);
 
   // If there was an invalid byte, find it and throw an exception.
   for (var i = start; i < end; i++) {
     var byte = bytes[i];
     if (byte >= 0 && byte <= 0xff) continue;
-    throw new FormatException("Invalid byte 0x${byte.toRadixString(16)}.",
+    throw new FormatException(
+        "Invalid byte ${byte < 0 ? "-" : ""}0x${byte.abs().toRadixString(16)}.",
         bytes, i);
   }
 
   throw 'unreachable';
 }
 
 /// Returns the ASCII/Unicode code unit corresponding to the hexadecimal digit
 /// [digit].
 int _codeUnitForDigit(int digit) => digit < 10 ? digit + $0 : digit + $a - 10;