Remove Expect from core library.

tests/corelib/strings_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";
+
 // Dart test program for testing class 'Strings'.
 
 class StringsTest {
 
   StringsTest() {}
 
   toString() {
     return "Strings Tester";
   }
   static testCreation() {
     String s = "Hello";
     List<int> l = new List(s.length);
     for (int i = 0; i < l.length; i++) {
       l[i] = s.codeUnitAt(i);
     }
     String s2 = new String.fromCharCodes(l);
     Expect.equals(s, s2);
   }
 
   static void testMain() {
     testCreation();
   }
 }
 
 
 main() {
   StringsTest.testMain();
 }