Revert "Remove Expect from core library."

tests/language/raw_string_test.dart

 // 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.
 
-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
@@ skipping 16 matching lines @@
     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();
 }