Revert "Remove Expect from core library."

tests/language/context_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 for capturing.
 
-import "package:expect/expect.dart";
-
 class ContextTest {
   static foo(Function f) {
     return f();
   }
 
   static void testMain() {
     int x = 42;
     bar() { return x; }
     x++;
     Expect.equals(43, foo(bar));
   }
 }
 
 main() {
   ContextTest.testMain();
 }