Revert "Remove Expect from core library."

tests/compiler/dart2js_extra/interface_type_optimization_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";
-
 main() {
   var a = new A();
   Expect.equals(42, a.foo(42));
   var b = new B();
   Expect.equals(42 - 87, b.foo(42));
 
   // Make sure we do not try to track the parameter type of the
   // argument passed to test.
   Expect.equals("is !A", test(0));
   Expect.equals("is !A", test("fisk"));
@@ skipping 16 matching lines @@
   }
 }
 
 class A {
   foo(var x) => x;
 }
 
 class B implements A {
   foo(var x) => x - 87;
 }