esri.layers.FeatureLayer is not a constructor

I received this error when using the ArcGIS JavaScript API. After doing some digging, it turned out that esri.layers.FeatureLayer was undefined (because the API was not loaded) when I was calling it.

Al the API examples show that dojo.addOnLoad should be used but I was using Ext.onReady (another JS API). This was causing the problem because when Ext.onReady was executed the AGS JS API was not loaded. I had to use dojo.addOnLoad to make sure that when I call esri.layers.FeatureLayer it is actually loaded.