Remove Expect from core library.

tests/standalone/io/file_output_stream_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.
 // Testing file input stream, VM-only, standalone test.
 
+import "package:expect/expect.dart";
 import "dart:io";
 import "dart:isolate";
 
 void testOpenOutputStreamSync() {
   Directory tempDirectory = new Directory('').createTempSync();
 
   // Create a port for waiting on the final result of this test.
   ReceivePort done = new ReceivePort();
   done.receive((message, replyTo) {
     tempDirectory.deleteSync();
@@ skipping 81 matching lines @@
   };
   x.close();
   x.onError = (e) => Expect.fail("No error expected");
 }
 
 main() {
   testOpenOutputStreamSync();
   testOutputStreamNoPendingWrite();
   testOutputStreamFlush();
 }