In certain cases it is not possible to downcast `IConstruct` to `Construct`

[eladb]

The way nodes are associated with constructs breaks a use case where IConstruct objects would be treated as Construct objects through downcasting.

See aws/aws-cdk#6885

Granted, down-casting is against the "contract" of the API, but it still exists in a few places in the AWS CDK is already a stable product, and therefore we have to figure out a way to enable this use case.

[eladb]

Since this is only a single case in the AWS CDK that depends on this, we decided to simply fix it for now.

[eladb]

This only fails if the class that implements IConstruct is not a "real" construct, which is currently not a supported use case.