Revert "Remove Expect from core library."

tests/language/optimized_constant_array_string_access_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";
-
 // Test optimized constant string and constant array access.
 
 int testConstantStringAndIndexCodeUnitAt() {
   int test(b) {
     if (b) return "hest".codeUnitAt(400);
     return "hest".codeUnitAt(2);
   }
 
   Expect.throws(() => test(true));
   for (int i = 0; i < 10000; i++) test(false);
@@ skipping 25 matching lines @@
   for (int i = 0; i < 10000; i++) { foo(1); }
   Expect.throws(() => foo(2));
 }
 
 
 main() {
   testConstantStringAndIndexCodeUnitAt();
   testConstantArrayAndIndexAt();
   testNonSmiIndex();
 }