[Sidenav] Issue when container and sidenav in different components

#### Bug, feature request, or proposal:
I have the template like this:
``
` `
`...`
``

`` is:
``
` ...`
``

When I have my sidenav in two components like this, the mode "**side**" is not working anymore. The mode switch to "**over**".
When I have all in the same component, it's working.

I think it's due to a another tag between and and it break the css rules.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular Material 5.0.1

首先使用 Angular Material 5.0.1 重现报告中的模板，将 mat-sidenav-container 和 mat-sidenav 放置在不同的组件中，然后将其与放置在同一组件中的安排进行比较。当 sidenav 嵌套在 my-component 中时，mode="side" 仍然有效，而不是切换为 mode="over"，即表示完成。

由索引模型根据 Issue 内容生成。