null safety upgrade

[jcramer]

Just wanted to help moving things along for this. I know there is another PR for null safety but there are conflicts and it may be getting stale. I did a quick upgrade from master and checked the tests are still passing.

It looks like vscode dart plugin thinks lines were getting too long, so it automatically did some linting to multi-lines in some cases. If this is undesirable, lmk and I'll try to revert it.

On a separate note, I noticed travis build hasn't been setup for this repo. Would be as simple as adding .travis.yml file with just language: dart.

[SFzxc]

Thanks @jcramer. Would you mind to become maintainer of https://github.com/dart-bitcoin/bitcoin_flutter

[SFzxc]

This cool PR is pre-step to resolved dart-bitcoin/bitcoin_flutter#50. let me know if you're interesting, Thank for your contributing!

[p3root]

@SFzxc will look into it!