Solution for some of Flex programming problems: The class not found in jar file: mxmlc.jar

This error may occur when you switch from Flex 3 SDK to Flex 4 SDK.

To resolve this error,
copy the {FLEX_HOME}/ant/lib/flexTasks.jar file from Flex 4 SDK and replace the existing flexTasks.jar in  Ant's lib directory ({ANT_HOME}/lib)