Revert "Remove Expect from core library."

tests/language/implied_interface_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 BaseClass {
   var foo;
   BaseClass() { foo = 0; }
   toString() => "BaseClass";
 }
 
 /**
  * This class declaration causes an intentional type warning as it
  * isn't marked abstract. It is abstract because it doesn't
  * "implement" the field foo.
@@ skipping 32 matching lines @@
   Expect.equals(true, c1 is !ExtendsClass);
   Expect.equals(true, c2 is BaseClass);
   Expect.equals(true, c2 is ExtendsClass);
   Expect.equals(true, c2 is !ImplementsClass);
   Expect.equals("BaseClass", "${new BaseClass()}");
 
   // Verify we don't inherit toString from BaseClass
   Expect.notEquals("BaseClass", "${c1}");
   Expect.notEquals("BaseClass", "${c2}");
 }