Remove Expect from core library.

tests/compiler/dart2js_extra/closure_capture3_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";
+
 class Closure {
   var x;
   Closure(val) : this.x = val;
 
   foo() {
     return fun() {
       return x;
     };
   }
 
@@ skipping 13 matching lines @@
   }
 }
 
 main() {
   var c = new Closure(499);
   var f = c.foo();
   Expect.equals(499, f());
   Expect.equals(500, c.bar()());
   Expect.equals(501, c.nestedClosure()());
 }