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._timerMillisecondClock"

[zra, 2015/12/10 22:53:46]

Other than the call site, this is the only place it is documented how this field
should be populated by the embedder. Maybe a little more guidance here would be
appropriate. Maybe just a function type like () -> int would be enough.

[rmacnak, 2015/12/11 18:39:40]

Added.

Noting that eventHandlerSendData has a far more complicated undocumented API.

+  static var timerMillisecondClock;
   // Example: "dart:io _EventHandler._sendData"
   static var eventHandlerSendData;
   // 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";