Remove Expect from core library.

tests/corelib/string_buffer_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(srdjan): Move StringBuffer to visible names.
 
 class StringBufferTest {
   static testConstructor() {
     StringBuffer bf = new StringBuffer("");
     Expect.equals(true, bf.isEmpty);
 
     bf = new StringBuffer("abc");
     Expect.equals(3, bf.length);
     Expect.equals("abc", bf.toString());
@@ skipping 127 matching lines @@
     testAdd();
     testAddAll();
     testClear();
     testChaining();
   }
 }
 
 main() {
   StringBufferTest.testMain();
 }