Add ticks counter to Timer.

sdk/lib/_internal/js_runtime/lib/isolate_helper.dart

Index: sdk/lib/_internal/js_runtime/lib/isolate_helper.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/isolate_helper.dart b/sdk/lib/_internal/js_runtime/lib/isolate_helper.dart
index ebe79513d986733fccebc8d742528296905f7210..463858ab2faefed015975c8c52e1110d1f7668ba 100644
--- a/sdk/lib/_internal/js_runtime/lib/isolate_helper.dart
+++ b/sdk/lib/_internal/js_runtime/lib/isolate_helper.dart
@@ -1365,6 +1365,7 @@ class TimerImpl implements Timer {
   final bool _once;
   bool _inEventLoop = false;
   int _handle;
+  int _tick = 0;
 
   TimerImpl(int milliseconds, void callback()) : _once = true {
     if (milliseconds == 0 && (!hasTimer() || _globalState.isWorker)) {
@@ -1390,6 +1391,7 @@ class TimerImpl implements Timer {
       void internalCallback() {
         _handle = null;
         leaveJsAsync();
+        this._tick = 1;
         callback();
       }
 
@@ -1407,10 +1409,19 @@ class TimerImpl implements Timer {
       : _once = false {
     if (hasTimer()) {
       enterJsAsync();
+      int start = JS('int', 'Date.now()');
       _handle = JS(
           'int',
           'self.setInterval(#, #)',
           convertDartClosureToJS(() {
+            int tick = this._tick + 1;
+            if (milliseconds > 0) {
+              int duration = JS('int', 'Date.now()') - start;
+              if (duration > (tick + 1) * milliseconds) {
+                tick = duration ~/ milliseconds;
+              }
+            }
+            this._tick = tick;
             callback(this);
           }, 0),
           milliseconds);
@@ -1419,6 +1430,8 @@ class TimerImpl implements Timer {
     }
   }
 
+  int get tick => _tick;
+
   void cancel() {
     if (hasTimer()) {
       if (_inEventLoop) {