Revert "Remove Expect from core library."

tests/language/allocate_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";
-
 class MyAllocate {
   const MyAllocate([int value = 0]) : value_ = value;
   int getValue() { return value_; }
   final int value_;
 }
 
 class AllocateTest {
   static testMain() {
     Expect.equals(900, (new MyAllocate(900)).getValue());
   }
 }
 
 main() {
   AllocateTest.testMain();
 }