GWT-Compiler Validation Errors

Hello,

i just started with widgetset development. I created a Vaadin project and a widgetset with the eclipse plugin. When compiling the widgetset i get the following message:

Starting GWT compiler
Compiling module de.securedvaadin.FrameworktestsWidgetset
   Validating newly compiled units
      [ERROR] Errors in 'file:/Development/workspaces/default/vaadintests/src/main/java/de/securedvaadin/client/SimpleDiv.java'
         [ERROR] Line 14: No source code is available for type com.vaadin.ui.AbstractComponent; did you forget to inherit a required module?
         [ERROR] Line 22: No source code is available for type com.vaadin.terminal.PaintTarget; did you forget to inherit a required module?
         [ERROR] Line 22: No source code is available for type com.vaadin.terminal.PaintException; did you forget to inherit a required module?
03.02.2012 12:40:55 com.vaadin.terminal.gwt.widgetsetutils.ClassPathExplorer getPaintablesHavingWidgetAnnotation
INFO: Searching for paintables..
03.02.2012 12:40:56 com.vaadin.terminal.gwt.widgetsetutils.ClassPathExplorer getPaintablesHavingWidgetAnnotation
INFO: Search took 513ms
Widgetset compilation completed

I seems like everything worked well, but the errors are a little bit annoying (ok, there are only validation errors). But it is not my development philosophy to ignore errors easily. What can i do to prevent these errors from happening? And what causes them?

I googled for these things, but didn't found a good answer to this.

Greetings,
Marc