Revert "Remove Expect from core library."

tests/language/js_properties_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.
 // Test that JavaScript properties on Object can still be classes in
 // Dart.
 
-import "package:expect/expect.dart";
-
 void main() {
   Expect.equals(42, new __defineGetter__().hello());
   Expect.equals(42, new __defineSetter__().hello());
   Expect.equals(42, new __lookupGetter__().hello());
   Expect.equals(42, new __lookupSetter__().hello());
   Expect.equals(42, new constructor().hello());
   Expect.equals(42, new hasOwnProperty().hello());
   Expect.equals(42, new isPrototypeOf().hello());
   Expect.equals(42, new propertyIsEnumerable().hello());
   Expect.equals(42, new toLocaleString().hello());
   Expect.equals(42, new toString().hello());
   Expect.equals(42, new valueOf().hello());
 }
 
 class Hello {
   int hello() => 42;
 }
 
 class __defineGetter__ extends Hello {}
 class __defineSetter__ extends Hello {}
 class __lookupGetter__ extends Hello {}
 class __lookupSetter__ extends Hello {}
 class constructor extends Hello {}
 class hasOwnProperty extends Hello {}
 class isPrototypeOf extends Hello {}
 class propertyIsEnumerable extends Hello {}
 class toLocaleString extends Hello {}
 class toString extends Hello {}
 class valueOf extends Hello {}