Revert "Remove Expect from core library."

tests/language/implicit_this_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";
-
 abstract class Interface {
   final x;
 }
 
 abstract class Abstract implements Interface {
   String toString() => x.toString();
 }
 
 // This class does not implement "x" either, but it is not marked
 // abstract.
@@ skipping 21 matching lines @@
 void main() {
   var x = new Abstract(); /// 02: runtime error
   var y = new SubAbstract1(); /// 01: continued
   var z = new SubAbstract2();
   var a = new SubSubAbstract2(); /// 04: continued
   Expect.equals(x, x); /// 02: continued
   Expect.equals('7', new Concrete().toString());
   Expect.equals('42', new SubConcrete(42).toString());
   Expect.equals('7', new SubConcrete(new Concrete()).toString());
 }