Revert "Remove Expect from core library."

tests/language/optimized_setter_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 setter situations, testing special cases in optimizing compiler.
 
-import "package:expect/expect.dart";
-
 class A {
   int field = 0;
 }
 
 class B extends A {
 }
 
 
 void sameImplicitSetter() {
   oneTarget(var a, var v) {
@@ skipping 126 matching lines @@
 
 void main() {
   for (int i = 0; i< 2000; i++) {}
   sameImplicitSetter();
   setterNoFeedback();
   sameNotImplicitSetter();
 
   multiImplicitSetter();
   multiNotImplicitSetter();
 }