Compiling the source code [for ArcGIS Viewer for Flex] requires Adobe Flex 4.1 or above. This is to support Arabic and other right-to-left locales. Note that the API itself still works with 4.0, 4.1 as well as 4.5.
In version 2.4 of the Viewer, Adobe Flex 4.1 SDK is the minimum SDK version to compile with.
No, I am not trying to build a mobile application.
I just downloaded the Viewer source code instead of the API, and I still get the error message:
Could not resolve <s:ViewNavigatorApplication> to a component implementation.