Update handling of multiline strings to specification.

tests/language/multiline_newline_lf.dart

Index: tests/language/multiline_newline_lf.dart
diff --git a/tests/language/multiline_newline_lf.dart b/tests/language/multiline_newline_lf.dart
index 48b6901890087a1582fa230afe5c168179a53cd6..1775977904d44c1be50dc85eec914ad014851e30 100644
--- a/tests/language/multiline_newline_lf.dart
+++ b/tests/language/multiline_newline_lf.dart
@@ -13,3 +13,13 @@ var nonConstantMultilineString = """
 a
 b
 """;
+
+const constantRawMultilineString = r"""
+\a
+\b
+""";
+
+var nonConstantRawMultilineString = r"""
+\a
+\b
+""";