1046: Type was not found or was not a compile-time constant: ContentElement. TextLayoutFramework
[etc] ElementFormat. FontMetrics. TextElement. TextLine. Vector.
In this case the class was not found because I was specifying Flash Player 9 when the class was in Flash Player 10.
When I went into the Project Properties > Flex Compiler and changed the required version to 10.0.0 the errors went away.
Please note this is a case post. There are additional posts on this error.