Null safety migration

[jakemac53]

Sending this one out for a review even before creating the official branch just due to the size of it, and the relative complexity of things like the Equality classes.

Note that not all tests are passing right now, the wrapper_test.dart is broken but due to violations in the test itself, and the PriorityQueue and QueueList tests are broken due to what I believe is an sdk issue which I will file and link here once I confirm.

[jakemac53]

SDK issue is here dart-lang/sdk#41060 This is resolved

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