Fix scanning of unterminated strings

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 b4088b3d4fd33ab5c7efae6f9bb560ff197deb5f..16bfb059f569c25d7e5fa32851fd021090f3e6d9 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
@@ -213,7 +213,7 @@ class Utf8BytesScanner extends ArrayBasedScanner {
       TokenType type, int start, bool asciiOnly, String closingQuotes) {
     String source = StringToken.decodeUtf8(bytes, start, byteOffset, asciiOnly);
     return new SyntheticStringToken(
-        type, source + closingQuotes, start, source.length);
+        type, source + closingQuotes, tokenStart, source.length);
   }
 
   @override