Remove Expect from core library.

tests/language/state_mangling3_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";
+
 foo(state0) {
   if (state0 == null) return 0;
   var sum = 0;
   for (int i = 0; i < state0.length; i++) {
     sum += state0[i];
   }
   state0 = inscrutableId(state0);
   for (int i = 0; i < state0.length; i++) {
     sum += state0[i];
   }
@@ skipping 11 matching lines @@
   int length = 3;
   operator [](i) => 1;
 }
 
 main() {
   Expect.equals(12, foo([1, 2, 3]));
   if (inscrutableId(0) == 0) {
     Expect.equals(6, foo(new A()));
   }
 }