Revert "Remove Expect from core library."

tests/corelib/null_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.
 
-import "package:expect/expect.dart";
-
 // Test that Null inherits properties from Object.
 
 main() {
   var x;
 
   Expect.isTrue(x is Object);
   Expect.isTrue(x is dynamic);
   Expect.isTrue(x is! String);
   Expect.isTrue(x is! int);
 
   // These shouldn't throw.
   x.runtimeType;
   x.toString();
   x.hashCode;
 
   // operator== is inherited from Object. It's the same as identical.
   // It's not really testable.
   Expect.isTrue(identical(x, null));
   Expect.isTrue(x == null);
 
   // Methods can be closurized and yields the same result.
   var ts = x.toString;
   Expect.equals(null.toString(), ts());
 
   // noSuchMethod is tested in null_nosuchmethod_test.dart.
 }