I can't find the source code for the Android Development Tools Plugin for Eclipse. It seems to have been moved a few times, and when I google the question, the answers point to the wrong places. Also http://source.android.com/discuss seems to be down.
For Android open source project downloaded in May 2011, the source for the eclipse plugins was in <aosp_root>/sdk/eclipse/plugins
$ cd sdk/eclipse/plugins; ls
com.android.ide.eclipse.adt
com.android.ide.eclipse.hierarchyviewer
com.android.ide.eclipse.tests
com.android.ide.eclipse.ddms
com.android.ide.eclipse.pdt
com.android.ide.eclipse.traceview