Revert "Remove Expect from core library."

tests/standalone/assert_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_asserts
+//
 // Dart test program testing assert statements.
-import "package:expect/expect.dart";
 
 class AssertTest {
   static test() {
     try {
       assert(false);
       Expect.fail("Assertion 'false' didn't fail.");
     } on AssertionError catch (error) {
       Expect.equals("false", error.failedAssertion);
       int pos = error.url.lastIndexOf("/", error.url.length);
       if (pos == -1) {
@@ skipping 24 matching lines @@
 
   static testMain() {
     test();
     testClosure();
   }
 }
 
 main() {
   AssertTest.testMain();
 }