Revert "Remove Expect from core library."

tests/language/string_escapes_test.dart

 // Copyright (c) 2012, 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 StringEscapesTest {
 
   static testMain() {
     testDelimited();
     testFixed2();
     testFixed4();
     testEscapes();
     testLiteral();
   }
 
@@ skipping 41 matching lines @@
 
   static testLiteral() {
     String str = "\a\c\d\e\g\h\i\j\k\l\$\{\}\"";
     Expect.equals(r'acdeghijkl${}"', str);
   }
 }
 
 main() {
   StringEscapesTest.testMain();
 }