Handle backslashes and newlines when escaping strings.

dart/tests/corelib/safe_to_string_test.dart

Index: dart/tests/corelib/safe_to_string_test.dart
diff --git a/dart/tests/corelib/safe_to_string_test.dart b/dart/tests/corelib/safe_to_string_test.dart
index 9a9379f58aec222e16179a6380ee738f88028fb3..a5b111789bc7a14854a3b53b757491b9ee1a3bba 100644
--- a/dart/tests/corelib/safe_to_string_test.dart
+++ b/dart/tests/corelib/safe_to_string_test.dart
@@ -10,6 +10,9 @@ main() {
   Expect.stringEquals('"\'\'"', NoSuchMethodError.safeToString("''"));
   Expect.stringEquals(r'"\""', NoSuchMethodError.safeToString('"'));
   Expect.stringEquals(r'"\"\""', NoSuchMethodError.safeToString('""'));
+
+  Expect.stringEquals(r'"\\\"\n"', NoSuchMethodError.safeToString('\\"\n'));
+
   Expect.stringEquals('null', NoSuchMethodError.safeToString(null));
   Expect.stringEquals('true', NoSuchMethodError.safeToString(true));
   Expect.stringEquals('false', NoSuchMethodError.safeToString(false));