I believe this is a bug in Cesium. In Animation.js (https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Widgets/Animation/Animation.js#L373) there is a multiline string that escapes newlines by placing a trailing backslash instead of using string concatentation, which is the valid EcmaScript way to do multiline strings.
Using the trailing backslash became an issue for us when bundling Animation.js using WebPack. Ultimately what happened in the bundle file was the animation code was indented which added a tab to the beginning of each line in the multiline string and had the side effect of creating extra textNodes in the DOM. That changed the childNode indices on lines 383 - 390 in the code referenced above and ultimately caused a Developer Error.
Using the preferred string concatenation would fix the issue. I'd be happy to create a pull request.