The emoji characters render differently on different platforms: see the comparison chart. This depends more on the operating system than the browser. Here is a comparison of Pizza slice character:

Mac: Windows 7: Chrome OS:

(All from Chrome browser). And this is the character itself, rendered by the browser you are using currently.

🍕

This variety is hardly acceptable for online-based presentation, which are often delivered from a computer other than the one on which the presentation was prepared. At present, Google deals with this by using an SVG font (apparently, their own), for example this is their rendering of pizza slice:

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0.1875l-0.328125 -0.03125z" fill-rule="nonzero"></path>

The result:

This makes the Slides platform-independent, but at the cost of not having colorful (possibly proprietary) icons.

As Rubén suggested, inserting images works; you can copy-paste an image into a presentation, taking it, e.g., from fileformat pages.