Expansion Panel panel-content on animation end event

[KarenGrigoryan92]

Bug, feature request, or proposal:

Feature request

What is the expected behavior?

I have big content in my panel and it takes a while,(5 sec.) , and i want to show some animation while this opening

What is the current behavior?

The GUI is freezing when user click in Expansion panel header

What are the steps to reproduce?

ng add @angular/material

What is the use-case or motivation for changing an existing behavior?

For big content GUI is freezing, and no animation is showed that something is in process

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

Angular 6, material 2

Is there anything else we should know?

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