Re: [w3c/ServiceWorker] Handle asynchronous modules (top-level await). (#1444)

@syg commented on this pull request.



> +
+      : Input
+      :: |record|, a [=Module Record=]
+      :: |moduleMap|, a [=/module map=]
+      :: |base|, a [=/URL=]
+      :: |seen|, a [=/set=] of [=/URLs=]
+      : Output
+      :: a boolean
+
+      1. If |record| is not a [=Cyclic Module Record=], then:
+          1. Return false.
+      1. If |record|.\[[Async]] is true, then:
+          1. Return true.
+      1. [=list/For each=] string |requested| of |record|.\[[RequestedModules]],
+          1. Let |url| be the result of [=resolve a module specifier|resolving a module specifier=] given |base| and |requested|.
+          1. Assert: |url| is never failure, because [=resolve a module specifier|resolving a module specifier=] must have been previously successful with these same two arguments.

I don't grok the module loading stuff enough myself to be confident with any suggested fix. I don't think resolving a module specifier itself can fail, but that a module graph could have failed to Link(). In which case a requested module's url may not map to any module record in the module map.

Is there an observable error timing currently for a "SW has a TLA" error vs a linking error? My in-flight [CL](https://chromium-review.googlesource.com/c/chromium/src/+/2903690) in Chrome chooses to only check if a module graph has any TLAs in it if it successfully linked. If it failed to link, it doesn't check if it's async at all and let the linking error propagate.

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