Product Version = NetBeans IDE 8.1 Beta (Build 201507302201)
Operating System = Windows 8 version 6.2 running on amd64
Java; VM; Vendor = 1.8.0_45
Runtime = Java HotSpot(TM) 64-Bit Server VM 25.45-b02

I was running this test when the problem occurred:
http://services.netbeans.org/synergy/client/app/#/case/478/suite/1130/v/1

After creating the activator and installer, my project would no longer compile. Here is a sample of the compiler error:

Installer.java:10: error: cannot access SharedClassObject
public class Installer extends ModuleInstall {
  class file for org.openide.util.SharedClassObject not found
warning: No processor claimed any of these annotations: javax.annotation.Generated
Installer.java:12: error: method does not override or implement a method from a supertype
    @Override

I wondered where the problem lies, and so I started over:

1. I created a new Ant-based NetBeans Platform Application.
2. Added a new module (normal one, not OSGi) to it.
3. Compile - everything is OK.
4. Add an Action using the file wizard.
5. Compile - everything still OK.
6. Add an installer via the file wizard.
7. Compile - compile fails with the above error.