[dart] Explicitly convert property to integer

With the new engine (which we do not yet run on the builders), this
property seems to be returned as a floating-point number. This behavior
shows when doing a led run with '-modernize'.

Change-Id: I3dbbcb671fcd81180d27d3dfda8afa4fddfd4342
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/158440
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: Karl Klose <karlklose@google.com>
2 files changed