code: private OcniBlock extractNativeCode(TreeNode node) {
      int offset = node.getStartPosition();
      String text = unit.getSource().substring(offset, offset + node.getLength());
      Matcher m = OCNI_PATTERN.matcher(text);
      if (m.find()) {
        String typeStr = m.group(1);
        try {
          OcniType type = OcniType.fromString(typeStr);
          return new OcniBlock(type, m.group(2).trim());
        } catch (IllegalArgumentException e) {
          ErrorUtil.warning(node, "Unknown OCNI type: " + typeStr);
          return null;
        }
      }
      if (options.jsniWarnings() && hasJsni(text)) {
        ErrorUtil.warning(node, "JSNI comment found");
      }
      return null;
    }
docs: Returns text from within a source code range, where that text is
  surrounded by OCNI-like tokens ("/&#42;-[" and "]-&#42;/"), warning
  if JSNI delimiters are found instead.
  
  @param node The node in which to extract the native code.
  @return the extracted text between the OCNI delimiters, or null if
  a pair of JSNI delimiters aren't in the specified text range