function attachShadowBlock(connection, shadowState) {
    connection.setShadowState(shadowState);
    return connection.targetBlock();
  }
Create a parent block with an unconnected connection.
@param {Blockly.Connection} connection The connection to use.
@param {Blockly.serialization.blocks.State} shadowState The state for the
    shadow block.
@returns {Blockly.Block} The newly created shadow block.