improve fasta unterminated string recovery

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 0ab8ca2226dc925f8de2cdfa0a9835208ed72413..b4088b3d4fd33ab5c7efae6f9bb560ff197deb5f 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
@@ -6,7 +6,9 @@ library fasta.scanner.utf8_bytes_scanner;
 
 import 'dart:convert' show UNICODE_BOM_CHARACTER_RUNE, UTF8;
 
-import '../../scanner/token.dart' show TokenType;
+import '../../scanner/token.dart' show SyntheticStringToken, TokenType;
+
+import '../../scanner/token.dart' as analyzer show StringToken;
 
 import '../scanner.dart' show unicodeReplacementCharacter;
 
@@ -198,7 +200,8 @@ class Utf8BytesScanner extends ArrayBasedScanner {
   }
 
   @override
-  StringToken createSubstringToken(TokenType type, int start, bool asciiOnly,
+  analyzer.StringToken createSubstringToken(
+      TokenType type, int start, bool asciiOnly,
       [int extraOffset = 0]) {
     return new StringToken.fromUtf8Bytes(
         type, bytes, start, byteOffset + extraOffset, asciiOnly, tokenStart,
@@ -206,6 +209,14 @@ class Utf8BytesScanner extends ArrayBasedScanner {
   }
 
   @override
+  analyzer.StringToken createSyntheticSubstringToken(
+      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);
+  }
+
+  @override
   CommentToken createCommentToken(TokenType type, int start, bool asciiOnly,
       [int extraOffset = 0]) {
     return new CommentToken.fromUtf8Bytes(