Revert "Remove Expect from core library."

tests/language/dynamic_call_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.
 // Dart test program for testing dynamic calls.
 
-import "package:expect/expect.dart";
-
 class Helper {
   Helper() {}
   int foo(int i) {
     return i;
   }
 }
 
 class DynamicCallTest {
   static int testMain() {
     Helper obj = new Helper();
     Expect.equals(1, obj.foo(1));
   }
 }
 
 main() {
   DynamicCallTest.testMain();
 }