update invalid hex digit recovery

pkg/front_end/lib/src/fasta/scanner/recover.dart

Index: pkg/front_end/lib/src/fasta/scanner/recover.dart
diff --git a/pkg/front_end/lib/src/fasta/scanner/recover.dart b/pkg/front_end/lib/src/fasta/scanner/recover.dart
index e14fd2307b8e13771c3b7cfe3ac72a4e785864d7..70484c48cdd65ae7cb82fa615e8608890714450d 100644
--- a/pkg/front_end/lib/src/fasta/scanner/recover.dart
+++ b/pkg/front_end/lib/src/fasta/scanner/recover.dart
@@ -137,7 +137,7 @@ Token defaultRecoveryStrategy(
   }
 
   recoverHexDigit() {
-    return synthesizeToken(errorTail.charOffset, "-1", TokenType.INT)
+    return synthesizeToken(errorTail.charOffset, "0", TokenType.INT)
       ..next = errorTail.next;
   }