Add comment to async* support code

runtime/lib/core_patch.dart

Index: runtime/lib/core_patch.dart
diff --git a/runtime/lib/core_patch.dart b/runtime/lib/core_patch.dart
index 11e307af2fe9782aaf8f9f5063f5a55211f91fce..13f75138101ec8e25f46e870ff97a32daafee9b7 100644
--- a/runtime/lib/core_patch.dart
+++ b/runtime/lib/core_patch.dart
@@ -61,6 +61,7 @@ class _SyncIterator implements Iterator {
         yieldEachIterator = null;
       }
       isYieldEach = false;
+      // moveNextFn() will update the values of isYieldEach and _current.
       if (!moveNextFn(this)) {
         moveNextFn = null;
         _current = null;