Provide option to avoid tree walking during transpilation about deno_emit

[benStre]

We also faced this issue in our project and implemented a transpileIsolated function in our fork of deno_emit, which provides this exact functionality.

As an alternative, we also created a standalone TS transpiler based on deno_ast/deno_graph, that takes TS module code as an input and returns the transpiled JS code.

[yacinehmito]

This is complicated and requires implementing something that could be built in, but it does prevent walking the entire import tree, so it's the most efficient.

This seems to cover your use case very well. Is there a compelling reason to make this behaviour built-in?

Can you explain your use-case a bit more? Maybe this is something that we can expect other people to need, but it is hard to tell until we know the why behind the use case.

[jordanbtucker]

There are two use cases here.

1. Creating a JS library for browsers using Deno's tooling and ecosystem. While it may be standard to have one mod.ts file that re-exports everything, that isn't the only way to do things. In fact, I think that's bad practice for browser modules because it forces the browser to fetch all dependencies even if they're not needed for the current task. I'd rather just give deno_emit one directory and have it transpile all files inside it.
2. Creating a full-stack Deno web app. The browser scripts need to be transpiled from TS to JS, however I don't want to worry about which scripts are going to be entry points. Again, I just want to give deno_emit a directory and transpile all the files in it.

I suppose this could also apply to any project that has multiple entry points that rely on common code. For example, foo.ts ultimately relies on common.ts, and bar.ts ultimately relies on common.ts too. Both foo.ts and bar.ts are entry points, but since they both rely on common.ts, common.ts will get transpiled twice.

The behavior I'm proposing also mirrors tools like tsc and babel, which are transpilers, rather than webpack and esbuild, which are bundlers. In other words, it makes sense for deno_emit's bundle to accept entry points and walk the tree, but it doesn't make sense to me to have transpile walk the tree.