Remove Expect from core library.

tests/corelib/string_substring_test.dart

Index: tests/corelib/string_substring_test.dart
diff --git a/tests/corelib/string_substring_test.dart b/tests/corelib/string_substring_test.dart
index 0cdac66d419d45927868848bbeeb7714acc38e3c..7fd9091034d379b6bddffaef1f60732c7af7f00d 100644
--- a/tests/corelib/string_substring_test.dart
+++ b/tests/corelib/string_substring_test.dart
@@ -2,6 +2,8 @@
 // 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";
+
 main() {
   // Test that not providing an optional argument goes to the end.
   Expect.equals("".substring(0), "");