improve fasta interpolation recovery

pkg/front_end/lib/src/scanner/errors.dart

 // Copyright (c) 2016, 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.
 
 import 'package:front_end/src/base/errors.dart';
 import 'package:front_end/src/fasta/fasta_codes.dart';
 import 'package:front_end/src/fasta/scanner/error_token.dart';
 import 'package:front_end/src/scanner/token.dart' show Token;
 import 'package:front_end/src/fasta/scanner/token_constants.dart';
 
@@ skipping 76 matching lines @@
       charOffset--;
     }
     reportError(errorCode, charOffset, arguments);
   }
 
   var errorCode = token.errorCode;
   switch (errorCode.analyzerCode) {
     case "UNTERMINATED_STRING_LITERAL":
       // TODO(paulberry,ahe): Fasta reports the error location as the entire
       // string; analyzer expects the end of the string.
-      charOffset = endOffset;
+      charOffset = endOffset - 1;
       return _makeError(ScannerErrorCode.UNTERMINATED_STRING_LITERAL, null);
 
     case "UNTERMINATED_MULTI_LINE_COMMENT":
       // TODO(paulberry,ahe): Fasta reports the error location as the entire
       // comment; analyzer expects the end of the comment.
       charOffset = endOffset;
       return _makeError(ScannerErrorCode.UNTERMINATED_MULTI_LINE_COMMENT, null);
 
     case "MISSING_DIGIT":
       // TODO(paulberry,ahe): Fasta reports the error location as the entire
@@ skipping 29 matching lines @@
     token = token.next;
     // If we've found an EOF token, its charOffset indicates where the end of
     // the input is.
     if (token.isEof) return token.charOffset == charOffset;
     // If we've found a non-error token, then we know there is additional input
     // text after [charOffset].
     if (token.type.kind != BAD_INPUT_TOKEN) return false;
     // Otherwise keep looking.
   }
 }