Remove Expect from core library.

tests/corelib/string_substring_test.dart

 // Copyright (c) 2011, 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";
+
 main() {
   // Test that not providing an optional argument goes to the end.
   Expect.equals("".substring(0), "");
   Expect.throws(() => "".substring(1),
       (e) => e is RangeError);
   Expect.throws(() => "".substring(-1),
       (e) => e is RangeError);
 
   Expect.equals("abc".substring(0), "abc");
   Expect.equals("abc".substring(1), "bc");
@@ skipping 13 matching lines @@
 
   Expect.equals("abc".substring(0, null), "abc");
   Expect.equals("abc".substring(1, null), "bc");
   Expect.equals("abc".substring(2, null), "c");
   Expect.equals("abc".substring(3, null), "");
   Expect.throws(() => "abc".substring(4, null),
       (e) => e is RangeError);
   Expect.throws(() => "abc".substring(-1, null),
       (e) => e is RangeError);
 }