Null Safety Migration

[rayliverified]

No description provided.

[eroo36]

hello so i still didnt really understand the concept of the null safety or how to apply it and not even expert enough to make my own packages yet so i tried to check it but probably what i will tell here might not really help. I tested two web projects using nullsafety branch and didnt see any problem or error. Only things that i saw on the console are some warnings like:
"The operand can't be null, so the condition is always true. Remove the condition.dart(unnecessary_null_comparison) responsive_grid.dart line 61"
So these are only warnings so i guess they are not that important. Anyways i dont know how can i help here if you have suggestions how i can help let me know.

[brenodt]

I found these points that I was unsure about. Besides that, PR looks good to me.

[rayliverified]

Thank you for reviewing @brenodt .
I'll make the changes next week and also migrate the sample projects.
There's also some new development this week using the ResponsiveFramework to give me greater confidence that the null safety migration works as expected.

[rayliverified]

@eroo36 Thank you for testing and catching the asserts.