Revert "Remove Expect from core library."

tests/language/super_operator_test.dart

 // Copyright (c) 2011, 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.
 // Dart test for testing super operator calls
 
-import "package:expect/expect.dart";
-
 
 class A {
   String val = "";
   List things;
 
   A() : things = ['D', 'a', 'r', 't', 42];
 
   operator + (String s) {
     val = "${val}${s}";
     return this;
@@ skipping 53 matching lines @@
   a += "Tell";   // operator + of class B.
   Expect.equals("TellTell", a.val);
   Expect.equals("rr", a[2]);  // operator [] of class B.
 
   a[4] = 1;  // operator []= of class B.
   Expect.equals(43, a.things[4]);
   Expect.equals(86, a[4]);
 
   Expect.throws(testRegression6403);
 }