Revert "Remove Expect from core library."

tests/compiler/dart2js_extra/nan_negate_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";
-
 // Don't convert !(a op b) to (a neg-op b) when a or b might be NaN.
 test(double n) {
   // Force known type to double, preserves NaN.
   n = 0.0 + n;
   Expect.isFalse(n >= 0);
   Expect.isTrue(!(n < 0));
 
   Expect.isFalse(n <= 0);
   Expect.isTrue(!(n > 0));
 
@@ skipping 93 matching lines @@
 main() {
   test(hideConstant(27, double.NAN));
   testConstant();
 }
 
 double hideConstant(int n, double result) {
   if (n == 1) return result;
   if ((n & 1) == 0) return hideConstant(n >> 1, result);
   return hideConstant(3 * n + 1, result);
 }