Fix strong mode errors

There are _a lot_ of strong mode warnings/errors in bignum.

```none
$ dartanalyzer --version
dartanalyzer version 1.21.0-dev.8.0$ dartanalyzer --strong --no-hints lib/ |wc -l
     176
```

(And looks like 166 are errors.)

We should fix these, for any app that wants to use bignum and DDC.