Revert "Remove Expect from core library."

tests/compiler/dart2js/redundant_phi_eliminator_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 'compiler_helper.dart';
 
 const String TEST_ONE = r"""
 void foo(bar) {
   var toBeRemoved = 1;
   if (bar) {
   } else {
   }
   print(toBeRemoved);
 }
@@ skipping 15 matching lines @@
   String generated = compile(TEST_ONE, entry: 'foo');
   RegExp regexp = new RegExp("toBeRemoved");
   Expect.isTrue(!regexp.hasMatch(generated));
 
   generated = compile(TEST_TWO, entry: 'foo');
   regexp = new RegExp("toBeRemoved");
   Expect.isTrue(!regexp.hasMatch(generated));
   regexp = new RegExp("temp");
   Expect.isTrue(!regexp.hasMatch(generated));
 }