Fix scanning of unterminated strings

pkg/front_end/test/scanner_fasta_test.dart

Index: pkg/front_end/test/scanner_fasta_test.dart
diff --git a/pkg/front_end/test/scanner_fasta_test.dart b/pkg/front_end/test/scanner_fasta_test.dart
index dcf80823bdc1325fa574ffec53e54e81141200fb..01b1aa594e1a881273cd619bbd3130ea1c22edae 100644
--- a/pkg/front_end/test/scanner_fasta_test.dart
+++ b/pkg/front_end/test/scanner_fasta_test.dart
@@ -685,6 +685,47 @@ class ScannerTest_Fasta_Direct extends ScannerTest_Fasta_Base {
     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