Revert "Remove Expect from core library."

tests/language/optimization_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.
 // Test various optimizations and deoptimizations of optimizing compiler..
 
-import "package:expect/expect.dart";
-
 addThem(a, b) {
   return a + b;
 }
 
 isItInt(a) {
   return a is int;
 }
 
 doNeg(a) {
   return -a;
@@ skipping 72 matching lines @@
     doStore2(growable, 12);
     Expect.equals(12, growable[2]);
   }
 
   // Deoptimize.
   doStore1(growable, 8);
   Expect.equals(8, growable[1]);
   doStore2(fixed, 101);
   Expect.equals(101, fixed[2]);
 }