Support passthrough dynamic import, or basic code doesn't work

Please at least support a basic passthrough for dynamic import for Closure Compiler. In the current version, any presence of dynamic import at all causes a compilation failure. This causes even basic use cases of dynamic import to cause the entire compilation to fail.

Example in.js:

(async () =>
{
	await import("https://cdn.example.com/library.js");
	externalLibrary.someFunc();
})();

Command line:

java -jar ./closure-compiler.jar --js in.js --js_output_file out.js --compilation_level SIMPLE --formatting PRETTY_PRINT --language_in ECMASCRIPT_2020 --language_out ECMASCRIPT_2020

Observed result:

in.js:4:7: WARNING - [JSC_PARSE_ERROR] Parse error. This language feature is not currently supported by the compiler: Dynamic module import
  4|    await import("https://cdn.example.com/library.js");
              ^

ERROR - [JSC_FEATURES_NOT_SUPPORTED_BY_PASS] Attempted to run pass "markUntranspilableFeaturesAsRemoved" on input with features it does not support. Running pass anyway.
Unsupported features: [Dynamic module import]

1 error(s), 1 warning(s)

No output file is generated.

Expected result:

Just output the file with all transforms, but passthrough import(XYZ) -> import(XYZ)

This is blocking us using JavaScript modules at all in our code. We want to insert a dynamic import, but then Closure Compiler will refuse to process any of our code at all.

[brad4d]

@AshleyScirra do you feel this issue would be completely closed by the following behavior?

If the compiler’s output is set to at least ES_2020, then:

1. import(anything) will be left in the source code entirely unmodified.
2. Specifically, the compiler will not pay any attention at all to the value passed to import(), so it won’t notice if it looks like the path to a file that is included in the compilation.
3. The compiler will infer the type of all such expressions to be Promise<*>. (You must cast the result of the promise to some type the compiler knows about in order to do anything with it.)

[AshleyScirra]

I think #2770 is asking for full support. I’m only asking here for Closure Compiler not to fail compilation due to the presence of dynamic import, i.e. the minimum possible support. I think there’s basically three levels of support:

1. Fail compilation if feature exists (current state)
2. Ignore feature/pass through compilation without alteration (what I’m asking for here)
3. Full support (e.g. potentially taking in to account dead code elimination, bundling, transpilation, tree shaking etc) - as apparently described in issue #2770

I’d have thought passthrough is more or less trivial, whereas full support would involve more significant work.