Don't allow escaping newlines in single line strings.

dart/tests/language/src/StringEscape3NegativeTestHelper.dart

Index: dart/tests/language/src/StringEscape3NegativeTestHelper.dart
diff --git a/dart/tests/language/src/StringEscape3NegativeTestHelper.dart b/dart/tests/language/src/StringEscape3NegativeTestHelper.dart
new file mode 100644
index 0000000000000000000000000000000000000000..6b7461cc6bf080ca3fa32f39ce99e7ec54ae7a2e
--- /dev/null
+++ b/dart/tests/language/src/StringEscape3NegativeTestHelper.dart
@@ -0,0 +1,7 @@
+// Copyright (c) 2012, 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.
+
+// An empty library, imported by StringEscape3NegativeTest.dart.
+
+#library('empty');