improve fasta unterminated string recovery

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 b7b66d52045225716bdeb50bc54db4eaa54e8c44..2c90b52e08dc999aabe3e684b3656cac90a80a4a 100644
--- a/pkg/front_end/lib/src/fasta/scanner/string_scanner.dart
+++ b/pkg/front_end/lib/src/fasta/scanner/string_scanner.dart
@@ -4,7 +4,9 @@
 
 library dart2js.scanner.string_scanner;
 
-import '../../scanner/token.dart' show TokenType;
+import '../../scanner/token.dart' show SyntheticStringToken, TokenType;
+
+import '../../scanner/token.dart' as analyzer show StringToken;
 
 import 'array_based_scanner.dart' show ArrayBasedScanner;
 
@@ -45,7 +47,8 @@ class StringScanner extends ArrayBasedScanner {
   void handleUnicode(int startScanOffset) {}
 
   @override
-  StringToken createSubstringToken(TokenType type, int start, bool asciiOnly,
+  analyzer.StringToken createSubstringToken(
+      TokenType type, int start, bool asciiOnly,
       [int extraOffset = 0]) {
     return new StringToken.fromSubstring(
         type, string, start, scanOffset + extraOffset, tokenStart,
@@ -53,6 +56,14 @@ class StringScanner extends ArrayBasedScanner {
   }
 
   @override
+  analyzer.StringToken createSyntheticSubstringToken(
+      TokenType type, int start, bool asciiOnly, String closingQuotes) {
+    String source = string.substring(start, scanOffset);
+    return new SyntheticStringToken(
+        type, source + closingQuotes, start, source.length);
+  }
+
+  @override
   CommentToken createCommentToken(TokenType type, int start, bool asciiOnly,
       [int extraOffset = 0]) {
     return new CommentToken.fromSubstring(