mat-icon shows text before showing properly

#### Bug, feature request, or proposal:
Bug or feature request.

#### What is the expected behavior?
When using `home`, nothing should be displayed until the icon is loaded properly. As soon as the icon is loaded, it should render.

#### What is the current behavior?
Depending on the speed of your connection, it is possible, that you see the text (for example `home`) instead of the icon.

#### What are the steps to reproduce?

1. Start the Angular Material Icon example in [Stackblitz](https://stackblitz.com/angular/yogqmggjokx).
It might be better to open it in it's own tab.
2. Open the Developer-Tools (in Chrome) and under the `Network`-Tab choose a very slow network throttling profile. This is needed, as the sample app is very small and therefore the problem is not noticeable with normal connections.
3. Reload the page and wait until it is loaded. You will see something like this:
After some time it shows the icon:

#### What is the use-case or motivation for changing an existing behavior?
In an app I am currently working on, this happens pretty often. Even if I only change the route (which does not trigger a reload of the page) I often see the text of the icons before the icon is displayed properly, so it does not seem to be related to the loading of the icon font.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
I am using @angular/material 6.3.1 together with TypeScript 2.7.2 on a Windows 10 PC.
The problem occures at least on Chrome, Opera, Firefox and Edge.

Reproduziere das Problem aus dem verlinkten StackBlitz-Beispiel mit einem sehr langsamen Netzwerkprofil und konzentriere dich dabei auf die Komponente mat-icon und ihr Ladeverhalten. Bestätige die Anzeige des Zwischentexts und überprüfe, dass das abgeschlossene Verhalten den Icon-Text in allen betroffenen Browsern verborgen hält, bis das Icon bereit ist.

Vom Indexierungsmodell aus dem Issue-Text verfasst.