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

src/lib/tree/data-source/flat-data-source.ts のリンクされた MatTreeFlattener.flattenNodes 実装から始め、[] と Observable<[]> の子を混在させた構造のフラット化を再現します。これらの子がどのようにキューに追加されるかを追跡し、フラット化された結果が走査順序を保持していることを確認します。

索引モデルが issue の本文から書いたものです。