Use a monotonic clock in the implementation of Timer.

runtime/lib/internal_patch.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.
 
 patch List makeListFixedLength(List growableList)
     native "Internal_makeListFixedLength";
 
 patch List makeFixedListUnmodifiable(List fixedLengthList)
     native "Internal_makeFixedListUnmodifiable";
 
 class VMLibraryHooks {
   // Example: "dart:isolate _Timer._factory"
   static var timerFactory;
+
   // Example: "dart:io _EventHandler._sendData"
   static var eventHandlerSendData;
+
+  // A nullary closure that answers the current clock value in milliseconds.
+  // Example: "dart:io _EventHandler._timerMillisecondClock"
+  static var timerMillisecondClock;
+
   // Implementation of Resource.readAsBytes.
   static var resourceReadAsBytes;
+
   // Implementation of package root/map provision.
   static var getPackageRoot;
   static var getPackageMap;
 }
 
 patch class CodeUnits {
   static final int cid = ClassID.getID(new CodeUnits(""));
 }
 
 final bool is64Bit = _inquireIs64Bit();
 
 bool _inquireIs64Bit() native "Internal_inquireIs64Bit";