Revert "Remove Expect from core library."

tests/standalone/package/packages/package1.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("package1");
 
 #import('package2.dart', prefix: 'p1');
 #import('package2.dart', prefix: 'p2');
 #import('package:package2.dart', prefix: 'p3');
 
 main() {
-  if (!identical(p1.x, p2.x)) {
-    throw new Error("Not identical");
-  }
-  if (!identical(p1.x, p3.x)) {
-    throw new Error("Not identical");
-  }
+  Expect.identical(p1.x, p2.x);
+  Expect.identical(p1.x, p3.x);
 }