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This changes how non-async module loading works. In general, module loading is triggered by a v8 callback. We ask it to process a module (a <script type= module>) and then for every module that it depends on, we get a callback. This callback expects the nested v8.Module instance, so we need to load it then and there (as opposed to dynamic imports, where we only have to return a promise). Previously, we solved this by issuing a blocking HTTP get in each callback. The HTTP loop was able to continuing downloading already-queued resources, but if a module depended on 20 nested modules, we'd issue 20 blocking gets one after the other. Once a module is compiled, we can ask v8 for a list of its dependent module. We can them immediately start to download all of those modules. We then evaluate the original module, which will trigger our callback. At this point, we still need to block and wait for the response, but we've already started the download and it's much faster. Sure, for the first module, we might need to wait the same amount of time, but for the other 19, chances are by the time the callback executes, we already have it downloaded and ready.