Thank you Odoe,
I am using MXMLC coomand line only. I use a local config xml to link agslib-2.0-2010-06-25.swc. My SWF applications work OK. However, I can't use this approach to compile a widget.
One thing I observed was that the ESRI "HelloWorldWidget" SWF is 68K. The same widget the I compiled from the source code is 364K. Actually, all my SWF applications are about the same size. I figure it out that it might have something to do with agslib-2.0-2010-06-25.swc. I took out the local config xml, now MXMLC is stuck at IBaseWidget.as.
I recalled that there was a flexviewer-1.0.swc in the old SFV. I can't find the flexviewer swc , nor the developer's guide, in the 2.0 distribution package.
Your help is hightly appreciated.
Jonathan Feng