Make SourceString Iterable<int>.

frog/leg/scanner/byte_strings.dart

Index: frog/leg/scanner/byte_strings.dart
diff --git a/frog/leg/scanner/byte_strings.dart b/frog/leg/scanner/byte_strings.dart
index 194a7f83575bff2a3c7407b4509439b6ebaca50f..4d9f068b199f71815d632fbd4166c8a0cf05819d 100644
--- a/frog/leg/scanner/byte_strings.dart
+++ b/frog/leg/scanner/byte_strings.dart
@@ -15,12 +15,15 @@ class ByteString implements SourceString {
 
   abstract String get charset();
 
-  String toString() => new Utf8Decoder(bytes, offset, length).toString();
+  String toString() => new String.fromCharCodes(
+      new Utf8Decoder(bytes, offset, length).decodeRest());
 
   bool operator ==(other) {
     throw "should be overridden in subclass";
   }
 
+  Iterator<int> iterator() => new Utf8Decoder(bytes, offset, length);
+
   int hashCode() {
     if (_hashCode === null) {
       _hashCode = computeHashCode();