IntelliJ IDEA

Starting from the version 11 it's not possible to add PROJECT_FOLDER/src as it was described in the previously posted (by ctorx) link http://java.dzone.com/tips/gwt-development-intellij-idea. Once we add a source folder it appears as an "Empty library" and doesn't work. This is because IDEA treats it as a folder where should be compiled classes not sources.

But there is a workaround. To fix it we have to hack the module's IML file. Find the following lines in the IML file of your entry point module

<orderEntry type="module-library">
  <library>
    <CLASSES />
    <JAVADOC />
    <SOURCES>
      <root url="file://$MODULE_DIR$/src/main/java" />
    </SOURCES>
  </library>
</orderEntry>

and move your path into the CLASSES tag, so the entire thing looks like the following

<orderEntry type="module-library">
  <library>
    <CLASSES>
      <root url="file://$MODULE_DIR$/src/main/java" />
    </CLASSES>
    <JAVADOC />
    <SOURCES />
  </library>
</orderEntry>