Remove Expect from core library.

tests/language/constructor6_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.
 // Dart test program for constructors and initializers.
 
+import "package:expect/expect.dart";
+
 // Super initializer and super constructor body are executed in with the same
 // bindings.
 
 String trace = "";
 
 int E(int i) {
   trace = "$trace$i-";
   return i;
 }
 
@@ skipping 25 matching lines @@
 }
 
 class C extends B {
   C() : super(10, 20);
 }
 
 main() {
   var c = new C();
   Expect.equals("10-1001-20-1011-1111-1121-11-21-1131-", trace);
 }