Remove Expect from core library.

runtime/tests/vm/dart/optimized_stacktrace_test.dart

Index: runtime/tests/vm/dart/optimized_stacktrace_test.dart
diff --git a/runtime/tests/vm/dart/optimized_stacktrace_test.dart b/runtime/tests/vm/dart/optimized_stacktrace_test.dart
index c3e24ba634144cdc2eac6440f613c5f5b7a3d887..40413c57702cc0ea54d5abda815176de3e137ce4 100644
--- a/runtime/tests/vm/dart/optimized_stacktrace_test.dart
+++ b/runtime/tests/vm/dart/optimized_stacktrace_test.dart
@@ -1,8 +1,8 @@
 // Copyright (c) 2013, 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.
-
 // Test correct source positions in stack trace with optimized functions.
+import "package:expect/expect.dart";
 
 // (1) Test normal exception. 
 foo(x) => bar(x);