Revert "Remove Expect from core library."

tests/language/tree_shake_typed_selector_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.
 // Check that dart2js emits code for classes that implement another
 // class.
 
-import "package:expect/expect.dart";
-
 class A {
   factory A() => new B();
   foo() => 0;
 }
 
 class B implements A {
   foo() => 42;
 }
 
 main() {
   var a = new A();
   if (a is A) {
     Expect.equals(42, a.foo());
   } else {
     Expect.fail('Should not be here');
   }
 }