Revert "Remove Expect from core library."

tests/standalone/io/string_decoder_test.dart

 // Copyright (c) 2013, 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.
 
-import "package:expect/expect.dart";
 import "dart:async";
 import "dart:io";
 
 void test() {
   // Code point U+10FFFF is the largest code point supported by Dart.
   var controller = new StreamController();
   controller.add([0xf0, 0x90, 0x80, 0x80]);  // U+10000
   controller.add([0xf4, 0x8f, 0xbf, 0xbf]);  // U+10FFFF
   controller.add([0xf4, 0x90, 0x80, 0x80]);  // U+110000
   controller.add([0xfa, 0x80, 0x80, 0x80, 0x80]);  //  U+2000000
@@ skipping 40 matching lines @@
   invalid([0xff], 1);
   invalid([0xf0, 0xc0], 1);
   invalid([0xc0, 0x80], 1);
   invalid([0xfd, 0x80, 0x80], 3); // Unfinished encoding.
 }
 
 void main() {
   test();
   testInvalid();
 }