Revert "Remove Expect from core library."

tests/language/argument_definition_test.dart

 // Copyright (c) 2012, 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 argument definition test.
 
-import "package:expect/expect.dart";
-
 int test(int a, {int b: 2, int c: 3}) {
   int result = 0;
   ?b;
   ?result;  /// 01: compile-time error
   if (?a) {
     result += 100;
   }
   if (?b) {
     result += 20;
   }
@@ skipping 51 matching lines @@
 
     Expect.equals(100, closure_test(1)());
     Expect.equals(720, closure_test(1, b: 2)());
     Expect.equals(523, closure_test(1, b: 2, c: 3)());
     Expect.equals(703, closure_test(1, c: 3)());
 
     Expect.equals(true, tl_test(0));
     Expect.equals(false, tl_test());
   }
 }