Revert "Remove Expect from core library."

tests/language/field_optimization_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 = 0;
   foo() {
     x++;
   }
   toto() {
     x = 99;
   }
   bar(y) {
     x = y;
   }
 }
 
 class B {
   operator +(other) => "ok";
 }
 main() {
   var a = new A();
   a.foo();
   a.toto();
   a.bar("str");
   a.bar(new B());
   a.foo();
   Expect.equals("ok", a.x);
 }