Revert "Remove Expect from core library."

tests/language/field_optimization3_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 a = 0;
   var b = 0;
   foo() {
     var c = b + 27;
     for (var i = 0; i < 1 ; i++) {
       for (var j = 0; j < 1; j++) {
         Expect.equals(50, c + 23);
       }
     }
     return a > 0.2;
   }
   setA(value) { a = value; }
   setB(value) { b = value; }
   operator >(other) => other == 0.2;
 }
 
 main() {
   var a = new A();
   Expect.isFalse(a.foo());
   a.setA(new A());
   a.setB(0);
   Expect.isTrue(a.foo());
 }