java.lang.NoClassDefFoundError: org/netbeans/lib/nbjavac/services/NBClassReader

So, any thoughts on how I can build NetBeans and keep it up to date?

I used mercurial to download NetBeans 8.1 sources, made an edit and 
compiled/built the sources because it was the only way I could figure 
out how to do what I wanted.

Even though the sources, that would be main-silver, are, I believe, 
updated when I build them the resulting development version of NetBeans 
has plugins that want to be updated, I guess having to do with release 
and dev version numbers.

Now I had run updates before on this build and it breaks java and so 
recompiled and let it go but now NetBeans said there were 20 updates 
available so I thought i might try again and of course java is broken 
again, meaning opening an existing java project or creating a new one 
puts up the error:

java.lang.NoClassDefFoundError: 
org/netbeans/lib/nbjavac/services/NBClassReader
     at 
org.netbeans.modules.java.source.parsing.JavacParser.createJavacTask(JavacParser.java:824)
     at 
org.netbeans.modules.java.source.parsing.JavacParser.createJavacTask(JavacParser.java:709)
     at 
org.netbeans.modules.java.source.parsing.CompilationInfoImpl.getJavacTask(CompilationInfoImpl.java:396)
     at 
org.netbeans.modules.java.source.parsing.JavacParser.getResult(JavacParser.java:491)
     at 
org.netbeans.modules.java.source.parsing.JavacParser.getResult(JavacParser.java:163)
     at 
org.netbeans.modules.parsing.impl.TaskProcessor.callGetResult(TaskProcessor.java:631)
     at 
org.netbeans.modules.parsing.impl.SourceCache.getResult(SourceCache.java:262)
     at 
org.netbeans.modules.parsing.impl.TaskProcessor$RequestPerformer.run(TaskProcessor.java:798)
     at org.openide.util.lookup.Lookups.executeWith(Lookups.java:304)
     at 
org.netbeans.modules.parsing.impl.TaskProcessor$RequestPerformer.execute(TaskProcessor.java:725)
[catch] at 
org.netbeans.modules.parsing.impl.TaskProcessor$CompilationJob.run(TaskProcessor.java:686)
     at 
java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
     at java.util.concurrent.FutureTask.run(FutureTask.java:266)
     at 
org.openide.util.RequestProcessor$Task.run(RequestProcessor.java:1443)
     at 
org.netbeans.modules.openide.util.GlobalLookup.execute(GlobalLookup.java:68)
     at org.openide.util.lookup.Lookups.executeWith(Lookups.java:303)
     at 
org.openide.util.RequestProcessor$Processor.run(RequestProcessor.java:2058)

There is an existing report here: 
https://netbeans.org/bugzilla/show_bug.cgi?id=257482

Any thoughts on how I can build NetBeans and keep it up to date?