Use a monotonic clock in the implementation of Timer.

runtime/lib/internal_patch.dart

Index: runtime/lib/internal_patch.dart
diff --git a/runtime/lib/internal_patch.dart b/runtime/lib/internal_patch.dart
index 8e955061138b462d53aeeb43b91db4e9ccd9c7f6..163ec6ec2741abb86d42c06b1ed663e4697ad54b 100644
--- a/runtime/lib/internal_patch.dart
+++ b/runtime/lib/internal_patch.dart
@@ -11,10 +11,17 @@ patch List makeFixedListUnmodifiable(List fixedLengthList)
 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;