Merge null safety branch to master

[mit-mit]

From chatting with @jakemac53, the recommendation is to move null safety to the master branches now that we're prepping for beta.

[mit-mit]

Also, once this lands, would be great to roll this to the SDK DEPS.

[dcharkes]

Just to double check, do we want to release package versions through master, and stop maintaining a legacy branch? (Anyone wanting to use new features that get landed, for example #61, would have to opt in to NNBD?)

[jakemac53]

Note that users don't have to update to NNBD to use new features, but they do have to update to the >=2.12.x sdk

[dcharkes]

2.12 is only on beta right? not on stable?

I was under the impression we only publish dev releases for packages with SDK constraints that are a beta or dev branch. If we have a different policy with the NNBD migration, that is fine by me.

[jakemac53]

I was under the impression we only publish dev releases for packages with SDK constraints that are a beta or dev branch.

This is a pre-release, we just are using a more specific -nullsafety name for it instead of -dev

[dcharkes]

I see, so master branch, but -nullsafety.x version name. 👍

[dcharkes]

I published https://pub.dev/packages/ffi/versions/0.2.0-nullsafety.0