Revert "Remove Expect from core library."

tests/language/instanceof3_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 the instanceof operation.
 
-import "package:expect/expect.dart";
-
 // In the type test 'e is T', it is a run-time error if T does not denote a type
 // available in the current lexical scope.
 
 isCheckedMode() {
   try {
     var i = 1;
     String s = i;
     return false;
   } catch (e) {
     return true;
@@ skipping 68 matching lines @@
     Expect.isTrue(got_type_error == isCheckedMode());
   }
 }
 
 main() {
   // Repeat type checks so that inlined tests can be tested as well.
   for (int i = 0; i < 5; i++) {
     testAll();
   }
 }