I am trying to create a Problems view alternative by extending the MarkerSupportView, but I cannot find the plugin that defines it. I know MarkerSupportView only exists @since 3.4, but I use 3.6. I have searched the Eclipse plugins directory for a jar likely to contain the class, but still could not find it.
MarkerSupportView is in the org.eclipse.ui.ide plugin.
To find classes in the Eclipse plugins go to Preferences > Plug-in Development and check the Include all plug-ins from target in Java search. Once this is set the Navigate > Open Type... dialog will search all the plugins in your target platform for classes.
If you have the Eclipse source installed then Open Type will show you the class source.