Add comments for constants which replaced large integer literals

runtime/lib/typed_data_patch.dart

Index: runtime/lib/typed_data_patch.dart
diff --git a/runtime/lib/typed_data_patch.dart b/runtime/lib/typed_data_patch.dart
index 8486611a30febc0f2d9d88e018116d52affc30ae..24062520229ce845ace63004d95d6fdbd3bb4d18 100644
--- a/runtime/lib/typed_data_patch.dart
+++ b/runtime/lib/typed_data_patch.dart
@@ -3044,6 +3044,9 @@ int _toUint32(int value) {
   return value & 0xFFFFFFFF;
 }
 
+// Note: in --limit-ints-to-64-bits mode all integers are 64-bit already.
+// Still, it is harmless to apply _uint64Mask because (1 << 64) is 0 (all bits
+// are shifted out), so _uint64Mask is -1 (its bit pattern is 0xffffffffffffffff).
 const _uint64Mask = (1 << 64) - 1;
 
 int _toInt64(int value) {