function addDeletableBlocks(block) {
    if (block.isDeletable()) {
      deleteList = deleteList.concat(block.getDescendants(false));
    } else {
      var children = block.getChildren();
      for (var i = 0; i < children.length; i++) {
        addDeletableBlocks(children[i]);
      }
    }
  }
Build a list of all deletable blocks that are reachable from the given
list of top blocks.
@param {!Array.<!Blockly.BlockSvg>} topBlocks The list of top blocks on the
    workspace.
@return {!Array.<!Blockly.BlockSvg>} A list of deletable blocks on the
    workspace.
@private