generate synthetic and error tokens when missing digit

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

Index: pkg/front_end/lib/src/fasta/scanner/string_scanner.dart
diff --git a/pkg/front_end/lib/src/fasta/scanner/string_scanner.dart b/pkg/front_end/lib/src/fasta/scanner/string_scanner.dart
index 8e45e86cb0dbe23481e5aeb06883bdfd6106314e..5d868884c984b6822d864dd9fa0421e332731293 100644
--- a/pkg/front_end/lib/src/fasta/scanner/string_scanner.dart
+++ b/pkg/front_end/lib/src/fasta/scanner/string_scanner.dart
@@ -57,10 +57,10 @@ class StringScanner extends ArrayBasedScanner {
 
   @override
   analyzer.StringToken createSyntheticSubstringToken(
-      TokenType type, int start, bool asciiOnly, String closingQuotes) {
+      TokenType type, int start, bool asciiOnly, String syntheticChars) {
     String source = string.substring(start, scanOffset);
     return new SyntheticStringToken(
-        type, source + closingQuotes, tokenStart, source.length);
+        type, source + syntheticChars, tokenStart, source.length);
   }
 
   @override