Update handling of multiline strings to specification.

tests/language/multiline_newline_lf.dart

 // Copyright (c) 2015, 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 multiline_newline_lf;
 
 const constantMultilineString = """
 a
 b
 """;
 
 var nonConstantMultilineString = """
 a
 b
 """;
+
+const constantRawMultilineString = r"""
+\a
+\b
+""";
+
+var nonConstantRawMultilineString = r"""
+\a
+\b
+""";