code: @Nonnull
    public CollectingJSCodeProvider addFlattenedAt (@Nonnegative final int nIndex, @Nullable final IHasJSCode aProvider)
    {
      if (aProvider != null)
        if (aProvider instanceof CollectingJSCodeProvider)
          m_aList.addAll (nIndex, ((CollectingJSCodeProvider) aProvider).m_aList);
        else
          m_aList.add (nIndex, aProvider);
      return this;
    }
docs: Add JS code at the specified index but unwrapping any
  {@link CollectingJSCodeProvider} instances.
  
  @param nIndex
  The index where the element should be added. Should be &ge; 0.
  @param aProvider
  The JS code provider to be added. May be <code>null</code>.
  @return this for chaining