Revert "Remove Expect from core library."

tests/language/class_codegen_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";
-
 class A {
   A() : x = 3;
   foo() => x;
   var x;
 }
 
 class B extends A {
   bar() => 499;
 }
 
 class C extends A {
   bar() => 42;
 }
 
 main() {
   // We don't instantiate A, but the codegen still needs to emit (parts of) it
   // for inheritence purposes.
   var b = new B();
   var c = new C();
   Expect.equals(3, b.foo());
   Expect.equals(3, c.foo());
   Expect.equals(499, b.bar());
   Expect.equals(42, c.bar());
 }