Bug in calculation margin-top mat-grid-tile inside mat-grid

[LoganDupont]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

The last 2 grid-tiles should be positioned correctly next to each other and not overlapping the other tiles.

What is the current behavior?

Something goes wrong when grid-tiles are positioned in grid. Some grid-tiles are overlapping each other.

What are the steps to reproduce?

https://stackblitz.com/edit/angular-material2-issue-jm4ztb

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

The grid-tiles should be displayed correctly in grid.

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

Angular 6.0.3
Material 6.2.0
All browsers

Is there anything else we should know?

After some investigation I saw that the calculation for the margin-top is not set correctly.

[vivian-hu-zz]

fixed with PR #12980

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