Revert "Remove Expect from core library."

tests/language/map_literal2_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.
 // Test program for map literals.
 
-import "package:expect/expect.dart";
-
 int nextValCtr;
 
 get nextVal {
   return nextValCtr++;
 }
 
 main() {
   // Map literals with string interpolation in keys.
   nextValCtr = 0;
   var map = {"a$nextVal": "Grey", "a$nextVal": "Poupon" };
   Expect.equals(true, map.containsKey("a0"));
   Expect.equals(true, map.containsKey("a1"));
   Expect.equals("Grey", map["a0"]);
   Expect.equals("Poupon", map["a1"]);
 }