Add a percent-encoding converter.

lib/src/hex/encoder.dart

Index: lib/src/hex/encoder.dart
diff --git a/lib/src/hex/encoder.dart b/lib/src/hex/encoder.dart
index 7a974e3df16a09df4bb13edfe694e0ec88b66b9b..a9c66a5a255cb4401de03655001a0bfb2aea6501 100644
--- a/lib/src/hex/encoder.dart
+++ b/lib/src/hex/encoder.dart
@@ -75,7 +75,8 @@ String _convert(List<int> bytes, int start, int end) {
   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);
   }