This change fixes the provider implementation of `Construct` for multi-lang components written in Node.js to wait for any in-flight RPCs to finish before returning the results, s.t. all registered child resources are created.
In additional, invocations of `construct` are now serialized so that each call runs one after another, avoiding concurrent runs, since `construct` modifies global state. We'll follow-up with a more general concurrency fix to allow nested `construct` calls within the same provider.
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