Revert "Remove Expect from core library."

tests/language/assertion_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.
 // VMOptions=--enable_type_checks
 //
 // Dart test program testing assert statements.
 
-import "package:expect/expect.dart";
-
 class AssertionTest {
   static testTrue() {
     int i = 0;
     try {
       assert(true);
     } on AssertionError catch (error) {
       i = 1;
     }
     return i;
   }
@@ skipping 48 matching lines @@
     Expect.equals(0, testTrue());
     Expect.equals(1, testFalse());
     Expect.equals(1, testClosure());
     Expect.equals(1, testClosure2());
   }
 }
 
 main() {
   AssertionTest.testMain();
 }