code: public void setWidgetMinSize(Widget child, int minSize) {
          assertIsChild(child);
          Splitter splitter = getAssociatedSplitter(child);
          // The splitter is null for the center element.
          if (splitter != null) {
              splitter.setMinSize(minSize);
          }
      }
docs: Sets the minimum allowable size for the given widget.
  
  <p>
  Its associated splitter cannot be dragged to a position that would make it
  smaller than this size. This method has no effect for the
  {@link DockLayoutPanel.Direction#CENTER} widget.
  </p>
  
  @param child the child whose minimum size will be set
  @param minSize the minimum size for this widget