code: private void convertSimpleObserverStringsToReferences(final PolymerClassDefinition cls) {
      for (MemberDefinition prop : cls.props) {
        if (prop.value.isObjectLit()) {
          Node observer = NodeUtil.getFirstPropMatchingKey(prop.value, "observer");
          if (observer != null && observer.isString()) {
            Node observerDirectReference =
                IR.getprop(cls.target.cloneTree(), "prototype", observer.getString())
                    .useSourceInfoFrom(observer);
  
            observer.replaceWith(observerDirectReference);
            compiler.reportChangeToEnclosingScope(observerDirectReference);
          }
        }
      }
    }
docs: Converts property observer strings to direct function references.
  
  <p>From: <code>observer: '_observerName'</code> To: <code>
  observer: ClassName.prototype._observerName</code>