title: Getting "java.lang.NoClassDefFoundError: org/eclipse/xtext/xbase/lib/Pair" with Eclipse Xtend
body: <p>I have no idea why I keep getting errors like these but I've been banging my head on the issues for a while now.</p>
  
  <p>Basically, I can't use the Xtend Library when I execute a project. Eclipse builds just fine, but during runtime if I execute a class that has a reference to one of Xtext's methods I get a NoClassDefinedError.</p>
  
  <p>For example this compiles just fine:
          var redirects = newHashMap(
              "/index.html" ->  "/"
          );</p>
  
  <p>But it does not execute because it has reference to Xtext's Pair.</p>
  
  <p>I feel like it has something to do with build automation. Maybe a Manifest or something but I'm fairly new to Eclipse/Java (and Xtend) so it's difficult for me to see where the issue lies.</p>
  
  <p>I started the java project out as a Google App Engine Web Application project and added the Xtend functionality afterwards. </p>
  
  <p>Anyone have any ideas?</p>