Revert "Remove Expect from core library."

tests/compiler/dart2js_extra/nan_negate_test.dart

Index: tests/compiler/dart2js_extra/nan_negate_test.dart
diff --git a/tests/compiler/dart2js_extra/nan_negate_test.dart b/tests/compiler/dart2js_extra/nan_negate_test.dart
index f5acd16101f7d89e7c024b4d9f4f6273b45e1578..1d86387da471e755415359f9bd0f1fda7e67771d 100644
--- a/tests/compiler/dart2js_extra/nan_negate_test.dart
+++ b/tests/compiler/dart2js_extra/nan_negate_test.dart
@@ -2,8 +2,6 @@
 // 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.