Nested <md-tab-group>s crash Angular

[oldchevy]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

Before this pull request animation behavior was working fine as intended:
fbfd032

What is the current behavior?

Nested <md-tab-group>s with more than two <md-tab>s crash the app. With two <md-tab>s the second tab is overlaid on top of the first.

What are the steps to reproduce?

Here is a plunker detailing the bug. The plunker is running on Angular v4.0.3:
https://plnkr.co/edit/qSZMIMlfSLIVcGkEzUQq

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

It should be possible to nest tab groupings.

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

Most recent, see PR referenced.

Is there anything else we should know?

[crisbeto]

Thanks for the report, I'll re-check it. Before I reverted #2411 via fbfd032, I did verify against the old test case, that it was supposed to fix (it was actually crashing the entire tab), and the workaround was no longer necessary.

[alexciesielski]

Dup #4271
Related #4255

(linking issues to close them more easily later)

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