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

Comienza en src/lib/tree/data-source/flat-data-source.ts, en la implementación enlazada de MatTreeFlattener.flattenNodes, y reproduce el aplanado de una estructura que mezcla [] con hijos Observable<[]>. Sigue cómo esos hijos se ponen en cola y verifica que el resultado aplanado preserve el orden de recorrido.

Escrito por el modelo de indexación a partir del texto del issue.