Function update

test/babel.js:57282–57299

(item, pattern)

	}

	function update(item, pattern) {
		// TODO: Test if memoizing `pattern` here is worth the effort.
		if (!pattern) {
			return;
		}
		var tree = parse(pattern, '');
		switch (tree.type) {
			case 'characterClass':
			case 'group':
			case 'value':
				// No wrapping needed.
				break;
			default:
				// Wrap the pattern in a non-capturing group.
				tree = wrap(tree, pattern);
		}
		assign(item, tree);
	}

	function wrap(tree, pattern) {
		// Wrap the pattern in a non-capturing group.