Revert "Remove Expect from core library."

tests/compiler/dart2js/begin_end_token_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.
 
-import "package:expect/expect.dart";
 import 'parser_helper.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/tree/tree.dart';
 
 void testNode(Node node, String expected, String text, [bool hard = true]) {
   var debug = 'text=$text,expected=$expected,node:${node}';
   Expect.isNotNull(node, debug);
 
   Token beginToken = node.getBeginToken();
   Expect.isNotNull(beginToken, debug);
   Token endToken = node.getEndToken();
@@ skipping 148 matching lines @@
   testNode(sendSet.assignmentOperator, "++", text);
   testNode(sendSet.arguments.head, "b", text);
 }
 
 void main() {
   testUnaryExpression();
   testAssignment();
   testIndex();
   testPostfix();
 }