MatTreeFlattener does not preserve traversal order while flattening mixed [] and Observable<[]>

When `MatTreeFlattener` `flattenNodes` method is used to flatten a data structure that mixes use of `[]` and `Observable<[]>`, the resulting flattened nodes are out of order as the children found from `Observable<[]>` are not done syncronously, but instead are done in order of discovery as they are each sent into the microtask queue to be done. They are sent in the correct order however to the task queue.

It appears the code in question is here:
https://github.com/angular/material2/blob/da63c6f85005966b42864a6c10f60896c8362361/src/lib/tree/data-source/flat-data-source.ts#L62-L68

Inizia in src/lib/tree/data-source/flat-data-source.ts, nell’implementazione collegata di MatTreeFlattener.flattenNodes, e riproduci l’appiattimento di una struttura che mescola [] con elementi figlio Observable<[]>. Traccia come questi elementi figlio vengono accodati e verifica che il risultato appiattito preservi l’ordine di attraversamento.

Scritto dal modello di indicizzazione a partire dal testo della issue.