Remove Expect from core library.

tests/corelib/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";
+
 // TODO(ngeoffray): test String methods with null arguments.
 class StringTest {
 
   static testMain() {
     testOutOfRange();
     testIllegalArgument();
     testConcat();
     testIndex();
     testCodeUnitAt();
     testEquals();
@@ skipping 265 matching lines @@
     }
     test("abc");
     test("");
     test(" ");
   }
 }
 
 main() {
   StringTest.testMain();
 }