_yaml: Move 'node_final_assertions' to 'Node._assert_fully_composited'

Benjamin Schubertrequested to merge
bschubert/node-api-noassertions into bschubert/new-node-api

Description

Move the node_final_assertions to a method on the Node class and rename it.

The rename is to make the method more clear on what it really does. Also make the method private, as we will only be using it internally.

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