addFlow( shaderStage, node ) {

		this.flowNodes[ shaderStage ].push( node );

		return node;

	}
Adds the Node to a target flow so that it can generate code in the 'generate' process.

@param {('vertex'|'fragment'|'compute')} shaderStage - The shader stage.
@param {Node} node - The node to add.
@return {Node} The node.