Add ticks counter to Timer.

pkg/dev_compiler/tool/input_sdk/private/isolate_helper.dart

Index: pkg/dev_compiler/tool/input_sdk/private/isolate_helper.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/private/isolate_helper.dart b/pkg/dev_compiler/tool/input_sdk/private/isolate_helper.dart
index 039b14f08a1321e2f099f4d6bf3ec9eedf872fde..42b18249d37d4a8f4216f251e9b1c61d385760fc 100644
--- a/pkg/dev_compiler/tool/input_sdk/private/isolate_helper.dart
+++ b/pkg/dev_compiler/tool/input_sdk/private/isolate_helper.dart
@@ -1355,6 +1355,7 @@ 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)) {
@@ -1380,6 +1381,7 @@ class TimerImpl implements Timer {
       void internalCallback() {
         _handle = null;
         leaveJsAsync();
+        _tick = 1;
         callback();
       }
 
@@ -1397,7 +1399,16 @@ class TimerImpl implements Timer {
       : _once = false {
     if (hasTimer()) {
       enterJsAsync();
+      int start = JS('int', 'Date.now()');
       _handle = JS('int', '#.setInterval(#, #)', global, () {
+        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);
       }, milliseconds);
     } else {
@@ -1405,6 +1416,8 @@ class TimerImpl implements Timer {
     }
   }
 
+  int get tick => _tick;
+
   void cancel() {
     if (hasTimer()) {
       if (_inEventLoop) {