Remove Expect from core library.

tests/language/field_optimization2_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";
+
 // Dart test program to test type-based optimization on fields.
 
 
 class A {
   var x;
   A() : x = new B();
   foo() {
     x++;
   }
 }
 
 class B {
   operator +(other) => 498;
 }
 main() {
   var a = new A();
   a.foo();
   a.foo();
   Expect.equals(499, a.x);
 }