Revert "Remove Expect from core library."

tests/corelib/json_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.
 
 library json_test;
 
-import "package:expect/expect.dart";
 import "dart:json";
 
 bool badFormat(e) => e is FormatException;
 
 void testJson(json, expected) {
   var value = parse(json);
   compare(expected, actual, path) {
     if (expected is List) {
       Expect.isTrue(actual is List);
       Expect.equals(expected.length, actual.length, "$path: List length");
@@ skipping 242 matching lines @@
 }
 
 main() {
   testNumbers();
   testStrings();
   testWords();
   testObjects();
   testArrays();
   testWhitespace();
 }