Revert "Remove Expect from core library."

tests/language/list_double_index_in_loop_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.
 // Dart test program for testing arrays.
 
-import "package:expect/expect.dart";
-
 bar() => true;
 
 // The type propagation in Dart2Js wrongly took the intersection of all incoming
 // types in a loop-phi. In this case the back-edge brought type 'number' which,
 // combined with 'integer' (i = 0) was narrowed to 'integer'. As a result no
 // check was inserted for the list access.
 foo(a) {
   var i = 0;
   while (true) {
     if (i > 1) return a[i];
     if (bar()) {
       // Adding a double guarantees a double result. Therefore guard by an if.
       i = i + 1.5;
     }
   }
 }
 
 main() {
   Expect.throws(() => foo([1, 2]));
 }