improve fasta unterminated string recovery

pkg/front_end/lib/src/fasta/scanner/array_based_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 fasta.scanner.array_based_scanner;
 
 import 'error_token.dart' show ErrorToken, UnmatchedToken;
 
 import '../../scanner/token.dart'
     show
         BeginToken,
         BeginTokenWithComment,
         Keyword,
         KeywordTokenWithComment,
         SyntheticToken,
         Token,
         TokenType,
         TokenWithComment;
 
-import 'token.dart' show StringToken;
+import '../../scanner/token.dart' as analyzer show StringToken;
 
 import 'token_constants.dart'
     show
         LT_TOKEN,
         OPEN_CURLY_BRACKET_TOKEN,
         OPEN_PAREN_TOKEN,
         STRING_INTERPOLATION_TOKEN;
 
 import 'characters.dart' show $LF, $STX;
 
@@ skipping 184 matching lines @@
       groupingStack.head.endGroup = tail;
       groupingStack = groupingStack.tail;
     }
   }
 
   void appendErrorToken(ErrorToken token) {
     hasErrors = true;
     appendToken(token);
   }
 
+  @override
   void appendSubstringToken(TokenType type, int start, bool asciiOnly,
       [int extraOffset = 0]) {
     appendToken(createSubstringToken(type, start, asciiOnly, extraOffset));
   }
 
+  @override
+  void appendSyntheticSubstringToken(
+      TokenType type, int start, bool asciiOnly, String closingQuotes) {
+    appendToken(
+        createSyntheticSubstringToken(type, start, asciiOnly, closingQuotes));
+  }
+
   /**
    * Returns a new substring from the scan offset [start] to the current
    * [scanOffset] plus the [extraOffset]. For example, if the current
    * scanOffset is 10, then [appendSubstringToken(5, -1)] will append the
    * substring string [5,9).
    *
    * Note that [extraOffset] can only be used if the covered character(s) are
    * known to be ASCII.
    */
-  StringToken createSubstringToken(TokenType type, int start, bool asciiOnly,
+  analyzer.StringToken createSubstringToken(
+      TokenType type, int start, bool asciiOnly,
       [int extraOffset = 0]);
 
   /**
+   * Returns a new synthetic substring from the scan offset [start]
+   * to the current [scanOffset] plus the [closingQuotes].
+   * The [closingQuotes] are appended to the unterminated string
+   * literal's lexeme but the returned token's length will *not* include
+   * those closing quotes so as to be true to the original source.
+   */
+  analyzer.StringToken createSyntheticSubstringToken(
+      TokenType type, int start, bool asciiOnly, String closingQuotes);
+
+  /**
    * This method is called to discard '<' from the "grouping" stack.
    *
    * [PartialParser.skipExpression] relies on the fact that we do not
    * create groups for stuff like:
    * [:a = b < c, d = e > f:].
    *
    * In other words, this method is called when the scanner recognizes
    * something which cannot possibly be part of a type parameter/argument
    * list, like the '=' in the above example.
    */
@@ skipping 62 matching lines @@
     //      |
     //     next
     //      v
     //     EOF
     TokenType type = closeBraceInfoFor(begin);
     appendToken(new SyntheticToken(type, tokenStart));
     begin.endGroup = tail;
     appendErrorToken(new UnmatchedToken(begin));
   }
 }