Revert "Remove Expect from core library."

tests/language/constant_propagation_phis_test.dart

 // Copyright (c) 2013, 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";
-
 // Test that constant propagation correctly updates phis when predecessor's
 // reachability changes.
 
 final keys = const ["keyA"];
 final values = const ["a"];
 
 main() {
   for (var i = 0; i < 10000; i++) test(keys[0]);
 }
 
 test(key) {
   var ref = key2value(key);
   Expect.equals("a", (ref == null) ? "-" : ref);
 }
 
 key2value(key) {
   var index = indexOf(keys, key);
   return (index == -1) ? null : values[index];
 }
 
 indexOf(keys, key) {
   for (var i = keys.length - 1; i >= 0; i--) {
     var equals = keys[i] == key;
     if (equals) return i;
   }
   return -1;
 }