Revert "Remove Expect from core library."

tests/language/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.
 // Replace with shared test once interface issues clarified.
 
-import "package:expect/expect.dart";
-
 class StringTest {
 
   static testMain() {
     testCodePoints();
     testNoSuchMethod();
     testStringsJoin();
     testCharCodes();
   }
 
   static testCodePoints() {
@@ skipping 26 matching lines @@
   static testCharCodes() {
     String s = new String.fromCharCodes(const [0x41, 0xC1, 0x424]);
     Expect.equals("A", s[0]);
     Expect.equals(0x424, s.codeUnitAt(2));
   }
 }
 
 main() {
   StringTest.testMain();
 }