Revert "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.
 
 void 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 159 matching lines @@
   testToString();
   testConstructor();
   testLength();
   testIsEmpty();
   testWrite();
   testWriteCharCode();
   testWriteAll();
   testClear();
   testChaining();
 }