Fix scanning of unterminated strings

pkg/front_end/test/scanner_fasta_test.dart

 // Copyright (c) 2017, 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 'dart:convert';
 
 import 'package:analyzer/src/fasta/token_utils.dart';
 import 'package:front_end/src/fasta/fasta_codes.dart';
 import 'package:front_end/src/fasta/scanner.dart' as usedForFuzzTesting;
 import 'package:front_end/src/fasta/scanner/error_token.dart' as fasta;
 import 'package:front_end/src/fasta/scanner/string_scanner.dart' as fasta;
@@ skipping 667 matching lines @@
     return createScanner(source, includeComments: true).tokenize();
   }
 
   void test_linestarts() {
     var scanner = createScanner("var\r\ni\n=\n1;\n");
     var token = scanner.tokenize();
     expect(token.lexeme, 'var');
     var lineStarts = scanner.lineStarts;
     expect(lineStarts, orderedEquals([0, 5, 7, 9, 12, 13]));
   }
+
+  void test_linestarts_synthetic_string() {
+    var scanner = createScanner("var\r\ns\n=\n'eh'\n'eh\n;\n");
+    Token firstToken = scanner.tokenize();
+    expect(firstToken.lexeme, 'var');
+    var lineStarts = scanner.lineStarts;
+    expect(lineStarts, orderedEquals([0, 5, 7, 9, 14, 18, 20, 21]));
+    var token = firstToken;
+    int index = 0;
+    while (!token.isEof) {
+      if (token is fasta.ErrorToken) {
+        expect(token.charOffset, 14,
+            reason: 'error token : $token, ${token.type}');
+        expect(token.charCount, 3,
+            reason: 'error token : $token, ${token.type}');
+      } else {
+        expect(token.charOffset, lineStarts[index],
+            reason: 'token # $index : $token, ${token.type}');
+        ++index;
+      }
+      token = token.next;
+    }
+  }
+
+  void test_linestarts_synthetic_string_utf8() {
+    var scanner = createScanner("var\r\ns\n=\n'éh'\n'éh\n;\n");
+    Token firstToken = scanner.tokenize();
+    expect(firstToken.lexeme, 'var');
+    var lineStarts = scanner.lineStarts;
+    expect(lineStarts, orderedEquals([0, 5, 7, 9, 14, 18, 20, 21]));
+    var token = firstToken;
+    int index = 0;
+    while (!token.isEof) {
+      if (token is! fasta.ErrorToken) {
+        expect(token.charOffset, lineStarts[index],
+            reason: 'token # $index : $token, ${token.type}');
+        ++index;
+      }
+      token = token.next;
+    }
+  }
 }
 
 /// Override of [ToAnalyzerTokenStreamConverter] that verifies that there are no
 /// errors.
 class ToAnalyzerTokenStreamConverter_NoErrors
     extends ToAnalyzerTokenStreamConverter {
   @override
   void reportError(
       ScannerErrorCode errorCode, int offset, List<Object> arguments) {
     fail('Unexpected error: $errorCode, $offset, $arguments');
   }
 }
 
 /// Override of [ToAnalyzerTokenStreamConverter] that records errors in an
 /// [ErrorListener].
 class ToAnalyzerTokenStreamConverter_WithListener
     extends ToAnalyzerTokenStreamConverter {
   final ErrorListener _listener;
 
   ToAnalyzerTokenStreamConverter_WithListener(this._listener);
 
   @override
   void reportError(
       ScannerErrorCode errorCode, int offset, List<Object> arguments) {
     _listener.errors.add(new TestError(offset, errorCode, arguments));
   }
 }