Migrate language_2/string to null safety.

Change-Id: Ib09f268c8a8edffba007cdd4962550a5e78ff808
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151090
Reviewed-by: Erik Ernst <eernst@google.com>
Commit-Queue: Bob Nystrom <rnystrom@google.com>
Auto-Submit: Bob Nystrom <rnystrom@google.com>

tests/language/string/raw_string_test.dart

@@ -0,0 +1,53 @@
+// Copyright (c) 2011, 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.
+
+// Note: This test relies on LF line endings in the source file.
+
+import "package:expect/expect.dart";
+
+class RawStringTest {
+  static testMain() {
+    Expect.equals("abcd", r"abcd");
+    Expect.equals("", r"");
+    Expect.equals("", r'');
+    Expect.equals("", r"""""");
+    Expect.equals("", r'''''');
+    Expect.equals("''''", r"''''");
+    Expect.equals('""""', r'""""');
+    Expect.equals(
+        "1\n2\n3",
+        r"""1
+2
+3""");
+    Expect.equals(
+        "1\n2\n3",
+        r'''1
+2
+3''');
+    Expect.equals(
+        "1",
+        r"""
+1""");
+    Expect.equals(
+        "1",
+        r'''
+1''');
+    Expect.equals("'", r"'");
+    Expect.equals('"', r'"');
+    Expect.equals("1", r"1");
+    Expect.equals("1", r"1");
+    Expect.equals("\$", r"$");
+    Expect.equals("\\", r"\");
+    Expect.equals("\\", r'\');
+    Expect.equals("\${12}", r"${12}");
+    Expect.equals("\\a\\b\\c\\d\\e\\f\\g\\h\\i\\j\\k\\l\\m",
+        r"\a\b\c\d\e\f\g\h\i\j\k\l\m");
+    Expect.equals("\\n\\o\\p\\q\\r\\s\\t\\u\\v\\w\\x\\y\\z",
+        r"\n\o\p\q\r\s\t\u\v\w\x\y\z");
+  }
+}
+
+main() {
+  RawStringTest.testMain();
+}