code: private void processForwardDeclare(Node n, Node parent) {
      CodingConvention convention = compiler.getCodingConvention();
  
      String typeDeclaration = null;
      try {
        typeDeclaration = Iterables.getOnlyElement(
            convention.identifyTypeDeclarationCall(n));
      } catch (NullPointerException | NoSuchElementException | IllegalArgumentException e) {
        compiler.report(
            JSError.make(
                n,
                INVALID_FORWARD_DECLARE,
                "A single type could not identified for the goog.forwardDeclare statement"));
      }
  
      if (typeDeclaration != null) {
        compiler.forwardDeclareType(typeDeclaration);
        // Forward declaration was recorded and we can remove the call.
        Node toRemove = parent.isExprResult() ? parent : parent.getParent();
        NodeUtil.deleteNode(toRemove, compiler);
      }
    }
docs: Process a goog.forwardDeclare() call and record the specified forward declaration.