emit comments into generated JS

[jmesserly]

idea: in a few methods like visitStatement, visitDeclaration, gather comments from the begin/end token stream and emit them in the desired location. This seems like a relatively simple way of preserving comments. We can then refine it over time to make locations more precise.

[jmesserly]

I looked at this a bit, and it doesn't seem easy to do with our current non-sourcemap generation technique. Probably best to revisit when we have better source location tracking

edit: the main issue was, we either have to drop comments if we don't know where they go, or put them in a somewhat arbitrary place, and either way it feels a bit confusing in the output.

[jmesserly]

This issue was moved to dart-lang/sdk#29347