Implement hashCode on objects stored in a set or used as map keys.

dart/sdk/lib/_internal/compiler/implementation/util/util.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.
 
 library dart2js.util;
 
 import "dart:collection";
 import 'util_implementation.dart';
 import 'characters.dart';
 
@@ skipping 92 matching lines @@
   for (int i = 0; i < string.length; i++) {
     int code = string.codeUnitAt(i);
     if (code < 0x20 || code == $DEL || code == $DQ || code == $LS ||
         code == $PS || code == $BACKSLASH || code >= 0x80) {
       writeEscapedOn(string, buffer);
       return;
     }
   }
   buffer.write(string);
 }
+
+int computeHashCode(part1, [part2, part3, part4]) {
+  return (part1.hashCode
+          ^ part2.hashCode
+          ^ part3.hashCode
+          ^ part4.hashCode) & 0x3fffffff;
+}