improve fasta unterminated string recovery

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

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 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;
 
 import 'token.dart' show CommentToken, DartDocToken, StringToken;
 
 /**
  * Scanner that reads from a String and creates tokens that points to
  * substrings.
  */
 class StringScanner extends ArrayBasedScanner {
@@ skipping 20 matching lines @@
   int advance() => string.codeUnitAt(++scanOffset);
   int peek() => string.codeUnitAt(scanOffset + 1);
 
   int get stringOffset => scanOffset;
 
   int currentAsUnicode(int next) => next;
 
   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,
         canonicalize: true, precedingComments: comments);
   }
 
   @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(
         type, string, start, scanOffset + extraOffset, tokenStart,
         canonicalize: true);
   }
 
   @override
   DartDocToken createDartDocToken(TokenType type, int start, bool asciiOnly,
       [int extraOffset = 0]) {
@@ skipping 14 matching lines @@
 
   SubStringScanner(this.baseOffset, String string,
       {bool includeComments: false})
       : super(string, includeComments: includeComments);
 
   @override
   void beginToken() {
     tokenStart = baseOffset + stringOffset;
   }
 }