Remove Expect from core library.

tests/corelib/string_base_vm_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.
 // Dart test program for testing class 'StringBase' (currently VM specific).
 
 library string_base_test;
+import "package:expect/expect.dart";
 
 class StringBaseTest {
 
   StringBaseTest() {}
 
   toString() {
     return "StringBase Tester";
   }
 
   static testInterpolation() {
@@ skipping 56 matching lines @@
   static void testMain() {
     testInterpolation();
     testCreation();
     testSubstring();
   }
 }
 
 main() {
   StringBaseTest.testMain();
 }