eliminate implicit casts/dynamic dispatch in DDC's SDK impl

[jmesserly]

Implicit casts and dynamic operations sometimes sneak into the generated code, causing a loss of performance. We could disable these to prevent them from being used in our private patch files/SDK implementation libraries. I'm unsure if the shared SDK code uses either of these, but we could specialize (via external) so DDC can implement those in a typed way.

EDIT: and to be clear, I'm mainly concerned with DDC's own SDK impl code. Because we're using a fair amount of JS builtins, it's easy to accidentally end up with implicit casts or dynamic where you didn't intend to do that.

[leafpetersen]

Yeah, this could definitely be useful. It's not far off of what @matanlurey was looking for with a flag to turn off dynamic operations as well - I'd bet if we could surface this in a nice way, then performance sensitive users might be interested in opting in to this.

It would be great to be able to audit the core SDK libraries in some way as well. I found some dynamic calls in json parsing by profiling, but I'm sure there's more lurking in there.

[jmesserly]

I've done a few passes of cleanups, but this would still be good to look into.

[jmesserly]

I had some progress in this CL, if someone wants to revive it https://dart-review.googlesource.com/c/sdk/+/77016