generate synthetic and error tokens when missing digit

pkg/front_end/lib/src/fasta/scanner/utf8_bytes_scanner.dart

Index: pkg/front_end/lib/src/fasta/scanner/utf8_bytes_scanner.dart
diff --git a/pkg/front_end/lib/src/fasta/scanner/utf8_bytes_scanner.dart b/pkg/front_end/lib/src/fasta/scanner/utf8_bytes_scanner.dart
index 16bfb059f569c25d7e5fa32851fd021090f3e6d9..6e87514d4988c0fb403209ae42186b94b5914457 100644
--- a/pkg/front_end/lib/src/fasta/scanner/utf8_bytes_scanner.dart
+++ b/pkg/front_end/lib/src/fasta/scanner/utf8_bytes_scanner.dart
@@ -210,10 +210,10 @@ class Utf8BytesScanner extends ArrayBasedScanner {
 
   @override
   analyzer.StringToken createSyntheticSubstringToken(
-      TokenType type, int start, bool asciiOnly, String closingQuotes) {
+      TokenType type, int start, bool asciiOnly, String syntheticChars) {
     String source = StringToken.decodeUtf8(bytes, start, byteOffset, asciiOnly);
     return new SyntheticStringToken(
-        type, source + closingQuotes, tokenStart, source.length);
+        type, source + syntheticChars, tokenStart, source.length);
   }
 
   @override