Fix bad refactoring in quote.dart.

pkg/fasta/lib/src/quote.dart

Index: pkg/fasta/lib/src/quote.dart
diff --git a/pkg/fasta/lib/src/quote.dart b/pkg/fasta/lib/src/quote.dart
index 6cf6969edc3ca644089b88e5c4779d45c3170223..62f98d702ac95c9ea583fcbbc438d46b26fe2131 100644
--- a/pkg/fasta/lib/src/quote.dart
+++ b/pkg/fasta/lib/src/quote.dart
@@ -218,10 +218,10 @@ String unescapeCodeUnits(List<int> codeUnits) {
           }
         } else {
           // Expect exactly 4 hex digits.
-          if (codeUnits.length < i + 4) return error(i, incompleteSequence);
+          if (codeUnits.length <= i + 4) return error(i, incompleteSequence);
           code = 0;
           for (int j = 0; j < 4; j++) {
-            int digit = codeUnits[i];
+            int digit = codeUnits[++i];
             if (!isHexDigit(digit)) return error(i, invalidCharacter);
             code = (code << 4) + hexDigitValue(digit);
           }